Properly track l-paren of a CXXFucntionalCastExpr.

In addition to storing more useful information in the AST, this
fixes a semantic check in template instantiation which checks whether
the l-paren location is valid.

Fixes PR16903.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188495 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/TreeTransform.h b/lib/Sema/TreeTransform.h
index b3e8720..9e57383 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -7317,7 +7317,7 @@
     return SemaRef.Owned(E);
 
   return getDerived().RebuildCXXFunctionalCastExpr(Type,
-                                      /*FIXME:*/E->getSubExpr()->getLocStart(),
+                                                   E->getLParenLoc(),
                                                    SubExpr.get(),
                                                    E->getRParenLoc());
 }