Previously, they were stored as localvarsyms in either the localsymtable
(for procedures/functions) or as localvarsyms in the staticsymtable (for
init/fini code of units/main programs). The latter was a hack (staticsymtables
normally cannot contain localvarsyms) and caused the temp allocator to also
allocate them as a local in fini code even if the default was only in the init
code.

The new approach ensures at most one copy gets allocated per unit, it doesn't
require explicit initialisation (since staticvarsyms are in bss -> zeroed by
default), gets rid of the localvarsyms in staticsymtables, and as a bonus
solves an issue with inconsistent LLVM debug information for the localvarsym
in init/fini code (since the staticsymtable is shared between the init and
fini code, so was the local, and therefore we generated debug info stating
it was defined in the fini code but within the scope of the init code).

Resolves #40395