These changes allow LLVM and Clang to be built with Clang targeting
Arm64EC using the MSVC linker.
Built with these options:
```
-DLLVM_ENABLE_PROJECTS="clang"
-DLLVM_HOST_TRIPLE=arm64ec-pc-windows-msvc
-DCMAKE_C_COMPILER=clang-cl.exe
-DCMAKE_C_COMPILER_TARGET=arm64ec-pc-windows-msvc
-DCMAKE_CXX_COMPILER=clang-cl.exe
-DCMAKE_CXX_COMPILER_TARGET=arm64ec-pc-windows-msvc
-DCMAKE_LINKER_TYPE=MSVC
```
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