optimize the 'StoredDeclsMap' for the common case where there is

exactly one decl with a specific name in a specific context.  This
avoids a bunch of malloc traffic and shrinks StoredDeclsMap to hold
one pointer instead of 3 words (for a std::vector).

This speeds up -fsyntax-only on cocoa.h with PTH by ~7.3%.

llvm-svn: 65103