[CMake] Unify llvm_check_linker_flag and llvm_check_compiler_linker_flag

These have the same purposes but two different implementations.
llvm_check_compiler_linker_flag uses CMAKE_REQUIRED_FLAGS which affects
flags used both for compilation and linking which is problematic because
some flags may be link-only and trigger unused argument warning when set
during compilation. llvm_check_linker_flag does not have this issue so
we chose it as the prevailaing implementation.

Differential Revision: https://reviews.llvm.org/D143052