[llvm] [runtimes] Append `-nostd*++` flags only for Clang (PR #151930)

[Michał Górny via llvm-commits]

mgorny wrote:

Ok, let's see if this works. I've combined check for clang with explicit checks to prevent the fallout from #108357.

I suppose another approach would be to test *both* the C and C++ compiler, and add the flags only if both work.

https://github.com/llvm/llvm-project/pull/151930