PR21565: Further refine the conditions for enabling eager parsing of

PR21565: Further refine the conditions for enabling eager parsing of
std::X::swap exception specifications (allowing parsing of non-conforming code
in libstdc++). The old conditions also matched the functions in MSVC's STL,
which were relying on deferred parsing here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222471 91177308-0d34-0410-b5e6-96231b3b80d8