[Sema] Fix using old initializer during switch statement transformation.

[vsapsai]

Summary

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

[ahatanak]

I think this is correct.

cfe/trunk/lib/Sema/TreeTransform.h

TreeTransform<Derived>::TransformSwitchStmt(SwitchStmt *S) {
   Sema::ConditionResult Cond = getDerived().TransformCondition(
       S->getSwitchLoc(), S->getConditionVariable(), S->getCond(),
       Sema::ConditionKind::Switch);
   if (Cond.isInvalid())
     return StmtError();
 
   // Rebuild the switch statement.
   StmtResult Switch
-    = getDerived().RebuildSwitchStmtStart(S->getSwitchLoc(),
-                                          S->getInit(), Cond);
+    = getDerived().RebuildSwitchStmtStart(S->getSwitchLoc(), Init.get(), Cond);
   if (Switch.isInvalid())
     return StmtError();
 
   // Transform the body of the switch statement.
   StmtResult Body = getDerived().TransformStmt(S->getBody());
   if (Body.isInvalid())
     return StmtError();
 

cfe/trunk/test/SemaCXX/cxx1z-init-statement-template.cpp

+// RUN: %clang_cc1 -std=c++1z -verify -emit-llvm-only %s
+// expected-no-diagnostics
+
+// rdar://problem/33888545
+template <unsigned int BUFFER_SIZE> class Buffer {};
+
+class A {
+public:
+  int status;
+};
+
+template <unsigned int N> A parse(Buffer<N> buffer);
+
+template<unsigned int N>
+void init_in_if(Buffer<N> buffer) {
+  if (A a = parse(buffer); a.status > 0) {
+  }
+}
+
+template<unsigned int N>
+void init_in_switch(Buffer<N> buffer) {
+  switch (A a = parse(buffer); a.status) {
+    default:
+      break;
+  }
+}
+
+void test() {
+  Buffer<10> buffer;
+  init_in_if(buffer);
+  init_in_switch(buffer);
+}