Implement ranking of standard conversion sequences by their qualification

conversions (e.g., comparing int* -> const int* against 
int* -> const volatile int*); see C++ 13.3.3.2p3 bullet 3.

Add Sema::UnwrapSimilarPointerTypes to simplify the control flow of
IsQualificationConversion and CompareQualificationConversion (and fix
the handling of the int* -> volatile int* conversion in the former).
 

llvm-svn: 57978