[clang] [flang] [llvm] [mlir] [polly] [CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH (PR #176420)

[via llvm-commits]

mcbarton wrote:

@aengelke I am part of an effort to enable plugin and LLVM_BUILD_LLVM_DYLIB support for Windows (see the following issue for more context https://github.com/llvm/llvm-project/issues/109483). By bisecting the commits on main I have identified this commit as the source of a regression we have in our build (specifically these linker errors https://github.com/mcbarton/ci-workflows/actions/runs/23344864570/job/67907422923#step:6:5899). If you do the following build for the commit before this PR went in (commit 30cd3d5a048c3e8e726263cd181f5cd13e759ff6) you'll see these don't occur

```
  cmake -S ./llvm/ -B build -G Ninja                `
        -DCMAKE_ASM_MASM_FLAGS="-m64"     `
        -DLLVM_ENABLE_PROJECTS="clang"              `
        -DLLVM_ENABLE_PLUGINS=On                    `
        -DLLVM_TARGETS_TO_BUILD="host"              `
        -DCMAKE_BUILD_TYPE=RelWithDebInfo           `
        -DCMAKE_CXX_COMPILER=clang-cl               `
        -DCMAKE_C_COMPILER=clang-cl                 `
        -DCMAKE_ASM_MASM_COMPILER=llvm-ml           `
        -DLLVM_BUILD_LLVM_DYLIB_VIS=On              `
        -DLLVM_LINK_LLVM_DYLIB=On                   `
        -DCLANG_ENABLE_STATIC_ANALYZER=OFF                  `
        -DLLVM_INCLUDE_BENCHMARKS=OFF                    `
        -DLLVM_INCLUDE_EXAMPLES=OFF                      `
        -DLLVM_INCLUDE_TESTS=OFF                         `
        -DLLVM_BUILD_TOOLS=OFF                           `
        -DCLANG_BUILD_TOOLS=OFF                          `
        -DCLANG_LINK_CLANG_DYLIB=On                                 
  cmake --build build --config RelWithDebInfo --parallel 4
```

cc @compnerd @vgvassilev

https://github.com/llvm/llvm-project/pull/176420