The Autoconf build already does this, but it was never ported to
CMake. The host linker version affects the flags that Clang pass
to the linker, notably whether it passes -demangle or not.

http://reviews.llvm.org/D6239

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221844 91177308-0d34-0410-b5e6-96231b3b80d8