Tighten up ASTContext::typesAreCompatible()...it needs to make sure the qualifiers match. The comment and C99 citation for this routine were correct...the code needed to conform to the comment/spec. This fixes the test added below.

Tightening up this routine forced tweaks to Sema::CheckSubtractionOperands() and Sema::CheckCompareOperands(). For example, they both need to operate on the unqualified pointee...

llvm-svn: 46522