[CMake] Make FindLibEdit.cmake more robust

FindLibEdit uses pkg-config to find the necessary flags, but this may break with cross-compilation,
because the PkgConfig module in CMake doesn't respect the SYSROOT specified in a toolchain file.
Instead of taking the parameters from pkg-config for granted, we check whether our compiler can
actually include and link against the library.

Fixes #55445
Fixes #55671

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D126450