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/Sema/SemaExprObjC.cpp b/clang/lib/Sema/SemaExprObjC.cpp
index 818a981..040cfdd 100644
--- a/clang/lib/Sema/SemaExprObjC.cpp
+++ b/clang/lib/Sema/SemaExprObjC.cpp
@@ -1097,12 +1097,7 @@
 
     // The type of @encode is the same as the type of the corresponding string,
     // which is an array type.
-    StrTy = Context.CharTy;
-    // A C++ string literal has a const-qualified element type (C++ 2.13.4p1).
-    if (getLangOpts().CPlusPlus || getLangOpts().ConstStrings)
-      StrTy.addConst();
-    StrTy = Context.getConstantArrayType(StrTy, llvm::APInt(32, Str.size()+1),
-                                         ArrayType::Normal, 0);
+    StrTy = Context.getStringLiteralArrayType(Context.CharTy, Str.size());
   }
 
   return new (Context) ObjCEncodeExpr(StrTy, EncodedTypeInfo, AtLoc, RParenLoc);