[Sema] Fix using old initializer during switch statement transformation.
It fixes a crash in CodeGen when we are trying to generate code for
initializer expression created before template instantiation, like
CallExpr '<dependent type>'
|-UnresolvedLookupExpr '<overloaded function type>' lvalue (ADL) = 'parse'
`-DeclRefExpr 'Buffer<N>' lvalue ParmVar 'buffer' 'Buffer<N>'
rdar://problem/
33888545
Reviewers: rsmith, ahatanak
Reviewed By: ahatanak
Subscribers: aemerson, kristof.beyls, cfe-commits
Differential Revision: https://reviews.llvm.org/D38009
llvm-svn: 313896