[AST] Squeeze some bits in LinkageComputer::QueryType

Replace the pair std::pair<const NamedDecl *, unsigned> where the
unsigned represents an LVComputationKind by a PointerIntPair.
This saves a pointer per entry in the map LinkageComputer::CachedLinkageInfo.

Differential Revision: https://reviews.llvm.org/D52268

Reviewed by: rjmccall, george.burgess.iv, erichkeane

llvm-svn: 342973