When caching code completions for global declarations, keep track of

the usage type of each declaration result, then compare those types to
the preferred type of the completion. This provides parity in the
priority calculation between the code-completion results produced
directly from Sema and those cached by ASTUnit. 

For the standard Cocoa.h (+ others) example, there's a penalty of 3-4
hundredeths of a second when caching the global results (for ~31,000
results), because we need an ASTContext-agnostic representation of
types for the comparison, and therefore we use... strings. Eventually,
we'd like to implement a more efficient ASTContext-agnostic encoding
of types.

llvm-svn: 111165