InstrProf: Don't emit coverage for uninstantiated templates

We include unused functions and methods in -fcoverage-mapping so that
we can differentiate between uninstrumented and unused. This can cause
problems for uninstantiated templates though, since they may involve
an incomplete type that can't be mangled. This shows up in things like
libc++'s <unordered_map> and makes coverage unusable.

Avoid the issue by skipping uninstantiated methods of a templated
class.