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

[Alexis Engelke via cfe-commits]

aengelke wrote:

I guess the suggestion is to add an option/env variable that suppresses automatic compilation database loading to Clang tools and use this in the tests. :/ — PCH really just exposes an underlying problem here, which is that the tests end up using whatever compile flags we happen to use to build LLVM.

As a temporary local workaround, most tools can be configured to look for compile commands in some directory, e.g. `clang-tidy -p <builddir>`.

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