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

[Slava Zakharin via llvm-commits]

vzakhari wrote:

> 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.

`clang++ -v -###` exits with `0` in my experiments.  Let me know if I missed something.

> 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.

`libflang_rt.quadmath` does abstract away the differences between platforms.  It does not provide the implementation itself, and instead relies on a concrete library providing implementation.  Ideally, we would have an implementation library that works for all targets, and then we do not need `libflang_rt.quadmath`.  I think there is no such functional library currently.

Note that this PR is about `REAL(16)` folding in the compiler itself.  But the issue is the same that we do not have a single library that works for all hosts.

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