Clean up: since we have FunctionDecl::IsInline, make it store the right value
for template instantiations, and use it to simplify the implementation of
FunctionDecl::isInlined().

This incidentally changes the result of isInlined on a declared-but-not-defined
non-inline member function from true to false. This is sort of a bug fix, but
currently isInlined is only called on function definitions, so it has no visible
effects.