Push nested-name-specifier source location information into

UnresolvedLookupExpr and UnresolvedMemberExpr.

Also, improve the computation that checks whether the base of a member
expression (either unresolved or dependent-scoped) is implicit. The
previous check didn't cover all of the cases we use in our
representation, which threw off source-location information for these
expressions (which, in turn, caused some breakage in libclang's token
annotation). 

llvm-svn: 126681