[llvm] [DirectX] Support ConstExpr GEPs (PR #150082)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

Build failure in:
```
  Clang :: CodeGenCXX/microsoft-abi-eh-disabled.cpp
  Clang :: CodeGenCXX/microsoft-abi-eh-ip2state.cpp
```
Is unrelated to this change.

https://github.com/llvm/llvm-project/pull/150082