Fixes #18. It's pretty easy to make this mistake when manually
transcribing C headers. :(

Fixes issue #19 by adding: LLVMCreateTypeAttribute; LLVMGetTypeAttributeValue; and LLVMIsTypeAttribute.

Add missing type attribute api.

Closes #19

See merge request taricorp/llvm-sys.rs!12

This adds support for attribute APIs that were sneakily added in
LLVM 12.0.1.

This is at least true for Debian Bullseye on my AMD64 system.

llvm-config --system-libs can return absolute paths