Implement __builtin_LINE() et. al. to support source location capture.

Summary:
This patch implements the source location builtins `__builtin_LINE(), `__builtin_FUNCTION()`, `__builtin_FILE()` and `__builtin_COLUMN()`. These builtins are needed to implement [`std::experimental::source_location`](https://rawgit.com/cplusplus/fundamentals-ts/v2/main.html#reflection.src_loc.creation).

With the exception of `__builtin_COLUMN`, GCC also implements these builtins, and Clangs behavior is intended to match as closely as possible. 

Reviewers: rsmith, joerg, aaron.ballman, bogner, majnemer, shafik, martong

Reviewed By: rsmith

Subscribers: rnkovacs, loskutov, riccibruno, mgorny, kunitoki, alexr, majnemer, hfinkel, cfe-commits

Differential Revision: https://reviews.llvm.org/D37035

llvm-svn: 360937
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index bbf7e04..870190a 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -907,13 +907,14 @@
 }
 
 CXXDefaultInitExpr::CXXDefaultInitExpr(const ASTContext &Ctx, SourceLocation Loc,
-                                       FieldDecl *Field, QualType Ty)
+                                       FieldDecl *Field, QualType Ty,
+                                       DeclContext *UsedContext)
     : Expr(CXXDefaultInitExprClass, Ty.getNonLValueExprType(Ctx),
            Ty->isLValueReferenceType() ? VK_LValue : Ty->isRValueReferenceType()
                                                         ? VK_XValue
                                                         : VK_RValue,
            /*FIXME*/ OK_Ordinary, false, false, false, false),
-      Field(Field) {
+      Field(Field), UsedContext(UsedContext) {
   CXXDefaultInitExprBits.Loc = Loc;
   assert(Field->hasInClassInitializer());
 }