Add two new functions to Qualifiers.

static Qualifiers removeCommonQualifiers(Qualifiers &L, Qualifiers &R)
Removes the common qualifiers from L and R and returns them in a new Qualifier.
This will be used in template diffing.

void removeQualifiers(Qualifiers Q)
Removes the qualifiers in Q from the current qualifier.
This replaces the current underlying implementation of operator- and
operator -= which only performed bit masking.

llvm-svn: 163752