[clang] [clang] Track final substitution for Subst* AST nodes (PR #132748)

[Pranav Kant via cfe-commits]

pranavk wrote:

With this commit, I get:
```
llvm-readelf -s -W reduced.o | grep -P 'optional.*aS.*srNS[0-9]_I'
     6: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND _ZN12_GLOBAL__N_18optionalaSIiTnNS_9enable_ifIXsr4_AndINS_17integral_constantIXu9__is_sameT_S0_EEEENS_7_OrImplIXsrNS3_IXu9__is_sameS4_iEEEE5valueEEEEE5valueEE4typeELi0EEEvS4_
     ```
     
Before this, the same symbol was:
```
     6: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND _ZN12_GLOBAL__N_18optionalaSIiTnNS_9enable_ifIXsr4_AndINS_17integral_constantIXu9__is_sameT_S0_EEEENS_7_OrImplIXsr10_IsNotSameIS4_iEE5valueEEEEE5valueEE4typeELi0EEEvS4_
```

https://github.com/llvm/llvm-project/pull/132748