[CMake] Factor out config prefix finding logic

[Ericson2314]

Summary

See the docs in the new function for details.

I think I found every instance of this copy pasted code. Polly could
also use it, but currently does something different, so I will save the
behavior change for a future revision.

We get the shared, non-installed CMake modules following the pattern
established in D116472.

It might be good to have LLD and Flang also use this, but that would be
a functional change and so I leave it as future work.

[Ericson2314]

Fix arg order error in Clang

[Ericson2314]

Oh, Flang and LLD are the same. Just do them now then

[Ericson2314]

Get MLIR too

[Ericson2314]

Oops, forgot some includes

[lebedev.ri]

Seems like a cleanup to me, and consistent with D116524.
Soft-accept, but only if D116524 is accepted first.

[beanz]

LGTM

[sebastian-ne]

llvm/CMakeLists.txt

209

Hi, adding this module path overwrites the llvm_check_linker_flag from llvm/cmake/modules/LLVMCheckLinkerFlag.cmake with the one defined in cmake/Modules/CheckLinkerFlag.cmake, which does not check the linker but the compiler.
This breaks our gcc+ld build because ld does not support --color-diagnostics.

Our downstream issue is here: https://github.com/GPUOpen-Drivers/llpc/issues/1645

[Ericson2314]

llvm/CMakeLists.txt

209

Thanks for letting me know. Let's just rename the /cmake one then; nothing there is installed, on purpose, so we can change it freely without worrying breaking an interface used downstream.

I will make a patch for that in the next 2 days if no one beats me to it.

[Ericson2314]

llvm/CMakeLists.txt

209

D117537 fixes this.