[flang] [llvm] [flang] Generate quadmath_wrapper.h for Flang Evaluate. (PR #132817)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> I believe the Clang command does not require the input source file, so I removed it as well.

It exits with an error if the file does not exist. It still prints the "selected GCC installation" before exiting and CMake seems to just ignore it unless using `COMMAND_ERROR_IS_FATAL`. So it should work, but I usually think that an erroring-out should indicate failure.

> For sure, the compiler needs a library that provides the same functionality as libquadmath. GNU libm provides it, though, not in all versions. I believe LLVM libm is not complete on this side yet.

Not all platforms are even based on GNU. Ideally, `libflang_rt.quadmath` should abstract away differences between platforms. If the environment does not support quadmath, then `libflang_rt.quadmath` should just be missing. Never should the compliler hardcode properties of the target environment. It can provide flags that indicate what the target environment will be.

https://github.com/llvm/llvm-project/pull/132817