Teach Clang to accept and ignore -f[no-]implicit-templates.

This GCC flag is useful when you want to control whether implicit
template instantiation occurs at the commandline level. Clang doesn't
currently support such controls, but technically *always* implicitly
instantiating (what Clang does, and what every other compiler still in
use does by default) is valid behavior even under
-fno-implicit-templates, it just may be slow and very wasteful. If
people really want this, we can try to implement it, but I don't have an
actual use.

This should help fix the build of libstdc++ with Clang, its build system
uses this flag.