[Clang][Sema] Narrow inline namespace filtration for unqualified friend declarations

rG04ba1856 introduced a call to FilterLookupForScope that is expensive
for very large translation units where it was observed to cause a 6%
compile time degradation. As the comment states, the only effect is to
"remove declarations found in inline namespaces for friend declarations
with unqualified names." This change limits the call to that scenario.
The test that was added by rG04ba1856 continues to pass, but the
observed degradation is cut in half.

Differential Revision: https://reviews.llvm.org/D135370