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/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 81ac44f..b7380aa 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -6801,8 +6801,12 @@
   if (!ToParamOrErr)
     return ToParamOrErr.takeError();
 
+  auto UsedContextOrErr = Importer.ImportContext(E->getUsedContext());
+  if (!UsedContextOrErr)
+    return UsedContextOrErr.takeError();
+
   return CXXDefaultArgExpr::Create(
-      Importer.getToContext(), *ToUsedLocOrErr, *ToParamOrErr);
+      Importer.getToContext(), *ToUsedLocOrErr, *ToParamOrErr, *UsedContextOrErr);
 }
 
 ExpectedStmt
@@ -7525,8 +7529,12 @@
   if (!ToFieldOrErr)
     return ToFieldOrErr.takeError();
 
+  auto UsedContextOrErr = Importer.ImportContext(E->getUsedContext());
+  if (!UsedContextOrErr)
+    return UsedContextOrErr.takeError();
+
   return CXXDefaultInitExpr::Create(
-      Importer.getToContext(), *ToBeginLocOrErr, *ToFieldOrErr);
+      Importer.getToContext(), *ToBeginLocOrErr, *ToFieldOrErr, *UsedContextOrErr);
 }
 
 ExpectedStmt ASTNodeImporter::VisitCXXNamedCastExpr(CXXNamedCastExpr *E) {