[CMake] Option to select C++ library for runtimes that use it

[phosek]

Summary

We currently have an option to select C++ ABI and C++ library for tests
but there are runtimes that use C++ library, specifically ORC and XRay,
which aren't covered by existing options. This change introduces a new
option to control the use of C++ libray for these runtimes.

Ideally, this option should become the default way to select C++ library
for all of compiler-rt replacing the existing options (the C++ ABI
option could remain as a hidden internal option).

[phosek]

I'm aware of the duplication of logic for handling of COMPILER_RT_CXX_LIBRARY and SANITIZER_TEST_CXX_LIBNAME. Right now my goal is to make the use of in-tree libc++ in compiler-rt explicit, and then cleaning up the handling logic in follow up changes.

[mstorsjo]

compiler-rt/CMakeLists.txt

534

Does this mean that if you set COMPILER_RT_CXX_LIBRARY=libcxx, it's assumed that you're actually _building_ that same libcxx in the same cmake invocation?

I'm currently building compiler-rt in a standalone cmake invocation, separate from all the others, with -DSANITIZER_CXX_ABI=libc++. Building them all at once might be something I could consider at some point, but I wouldn't want to force that situation right now, if it could be avoided?

[phosek]

compiler-rt/CMakeLists.txt

534

Yes, correct. This is modeled after https://github.com/llvm/llvm-project/blob/610139d2d9ce6746b3c617fb3e2f7886272d26ff/libcxx/CMakeLists.txt#L230 and https://github.com/llvm/llvm-project/blob/610139d2d9ce6746b3c617fb3e2f7886272d26ff/libcxx/cmake/Modules/HandleLibCXXABI.cmake#L109.

In your case you'd likely want to use -DCOMPILER_RT_CXX_LIBRARY=system-libcxx.

[mstorsjo]

LGTM

compiler-rt/CMakeLists.txt

534

Oh right, there was such an option too. Right, that'd work fine yeah - sorry for overlooking it.