Fix (nanopass `LET_SYNTAX`): use location from rhs instead of global one

Motivation and Context

Related Issue: #1664 (closed)

Description

This MR uses a different location for the compiled let_rhs. After the fix:

$ dune exec -- ligo compile contract curry.mligo --michelson-comments location --experimental-disable-optimizations-for-debugging
{ parameter unit ;       
  storage int ;
  code { {} ;
         { /* File "curry.mligo", line 1, character 0 to line 5, character 32 */
           { /* File "curry.mligo", line 1, character 0 to line 5, character 32 */
             LAMBDA
               (pair unit int)
               (pair (list operation) int)
               { {} ;
                 { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */
                   { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */ } ;
                   {} ;
                   { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */
                     { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */ } ;
                     CDR ;
                     {} ;
                     { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */
                       { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */ } ;
                       {} ;
                       { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */
                         { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */ } ;
                         {} ;
                         { /* File "curry.mligo", line 2, character 2 to line 5, character 32 */
                           { /* File "curry.mligo", line 2, characters 36-41 */
                             LAMBDA
                               (pair int int)
                               int
                               { {} ;
                                 UNPAIR ;
                                 {} ;
                                 {} ;
                                 SWAP ;
                                 {} ;
                                 { /* File "curry.mligo", line 2, characters 36-41 */
                                   { /* File "curry.mligo", line 2, characters 40-41 */ } ;
                                   { /* File "curry.mligo", line 2, characters 36-37 */ SWAP } ;
                                   ADD
                                   /* File "curry.mligo", line 2, characters 36-41 */ } } } ;
                           {} ;
                           { /* File "curry.mligo", line 3, character 2 to line 5, character 32 */
                             { /* File "curry.mligo", line 3, characters 10-12 */ PUSH int 42 } ;
                             {} ;
                             { /* File "curry.mligo", line 4, character 2 to line 5, character 32 */
                               { /* File "curry.mligo", line 4, characters 10-15 */
                                 { /* File "curry.mligo", line 4, characters 14-15 */ PUSH int 2 } ;
                                 { /* File "curry.mligo", line 4, characters 10-11 */ DIG 3 } ;
                                 ADD
                                 /* File "curry.mligo", line 4, characters 10-15 */ } ;
                               {} ;
                               { /* File "curry.mligo", line 5, characters 3-31 */
                                 { /* File "curry.mligo", line 5, characters 26-31 */
                                   { /* File "curry.mligo", line 5, characters 30-31 */ } ;
                                   { /* File "curry.mligo", line 5, characters 28-29 */ SWAP } ;
                                   PAIR ;
                                   { /* File "curry.mligo", line 5, characters 26-27 */ SWAP } ;
                                   SWAP ;
                                   EXEC } ;
                                 { /* File "curry.mligo", line 5, characters 3-24 */
                                   NIL operation
                                       /* File "curry.mligo", line 5, characters 3-24 */
                                   /* File "curry.mligo", line 5, characters 3-24 */ } ;
                                 PAIR } } } } } } } } } } ;
           {} } ;
         SWAP ;
         EXEC } }

The ADD gets location 2:36-41.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).