[libclc] [libclc] Fix libclc install on Windows when MSVC generator is used (PR #152703)

[Wenju He via cfe-commits]

wenju-he wrote:

> LGTM if you've tested it and it works 👍

Yes, the fix is verified both locally and in our internal CI.

I also fixed alias install in https://github.com/llvm/llvm-project/pull/152703/commits/b6cbefcbc06e42d7107723cb5b37749f3b1e0931. @frasercrmck please review this commit again, thanks. It fixes following install error when Ninja Multi-Config is used on Ubuntu 24.04 TLS:
```
CC=clang CXX=clang++ cmake -G "Ninja Multi-Config"  -DLLVM_USE_LINKER=lld -DLLVM_ENABLE_PROJECTS="llvm;clang;clang-tools-extra;libclc" -DLLVM_INCLUDE_TESTS=ON -DLLVM_BUILD_TESTS=ON -DLLVM_ENABLE_ASSERTIONS=ON ../llvm -DLLVM_TARGETS_TO_BUILD="X86;SPIRV;NVPTX;AMDGPU" -DLLVM_EXTERNAL_PROJECTS="llvm-spirv" -DCMAKE_INSTALL_PREFIX=install -DCMAKE_BUILD_TYPE=Release -DLIBCLC_TARGETS_TO_BUILD="all"
```
```
-- Installing: /llvm-project/build/install/share/clc/tahiti-amdgcn--.bc
CMake Error at tools/libclc/cmake_install.cmake:59 (file):
  file INSTALL cannot find
  "/llvm-project/build//lib/../lib/clang/22/lib/libclc/pitcairn-amdgcn--.bc":
  No such file or directory.
Call Stack (most recent call first):
  tools/cmake_install.cmake:517 (include)
  cmake_install.cmake:132 (include)


FAILED: [code=1] CMakeFiles/Debug/install.util
cd /llvm-project/build && /usr/bin/cmake/bin/cmake -DBUILD_TYPE=Debug -P cmake_install.cmake
ninja: build stopped: subcommand failed.
```

https://github.com/llvm/llvm-project/pull/152703