Add some unit tests for ClangASTContext.

In doing so, two bugs were uncovered (and fixed).  The first bug
is that ClangASTContext::RemoveFastQualifiers() was broken, and
was not removing fast qualifiers (or doing anything else for that
matter).  The second bug is that UnifyAccessSpecifiers treated
AS_None asymmetrically, which is probably an edge case, but seems
like a bug nonetheless.

llvm-svn: 265200