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/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 9d1526e..b8240d1 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -10208,6 +10208,31 @@
   return MaterializedTemporaryValues.lookup(E);
 }
 
+QualType ASTContext::getStringLiteralArrayType(QualType EltTy,
+                                               unsigned Length) const {
+  // A C++ string literal has a const-qualified element type (C++ 2.13.4p1).
+  if (getLangOpts().CPlusPlus || getLangOpts().ConstStrings)
+    EltTy = EltTy.withConst();
+
+  EltTy = adjustStringLiteralBaseType(EltTy);
+
+  // Get an array type for the string, according to C99 6.4.5. This includes
+  // the null terminator character.
+  return getConstantArrayType(EltTy, llvm::APInt(32, Length + 1),
+                              ArrayType::Normal, /*IndexTypeQuals*/ 0);
+}
+
+StringLiteral *
+ASTContext::getPredefinedStringLiteralFromCache(StringRef Key) const {
+  StringLiteral *&Result = StringLiteralCache[Key];
+  if (!Result)
+    Result = StringLiteral::Create(
+        *this, Key, StringLiteral::Ascii,
+        /*Pascal*/ false, getStringLiteralArrayType(CharTy, Key.size()),
+        SourceLocation());
+  return Result;
+}
+
 bool ASTContext::AtomicUsesUnsupportedLibcall(const AtomicExpr *E) const {
   const llvm::Triple &T = getTargetInfo().getTriple();
   if (!T.isOSDarwin())