Fix -fuse-ld feature detection error.

check_cxx_compiler_flag doesn't seem to try to link a program, so
the existing code doesn't correctly detect the availability of a given
linker.  This patch uses check_cxx_source_compiles instead.

I confirmed that cmake now reports this error

  Host compiler does not support '-fuse-ld=foo'

for -DLLVM_USE_LINKER=foo.

Differential Revision: https://reviews.llvm.org/D39274

llvm-svn: 316958