[AST][NFC] Pass the AST context to one of the ctor of DeclRefExpr.

All of the other constructors already take a reference to the AST context.
This avoids calling Decl::getASTContext in most cases. Additionally move
the definition of the constructor from Expr.h to Expr.cpp since it is calling
DeclRefExpr::computeDependence. NFC.

llvm-svn: 349901
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index a8e3b85..9fa3996 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1415,7 +1415,8 @@
   EnterExpressionEvaluationContext scope(
       S, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
   SourceLocation Loc = VD->getLocation();
-  Expr *VarRef = new (S.Context) DeclRefExpr(VD, false, T, VK_LValue, Loc);
+  Expr *VarRef =
+      new (S.Context) DeclRefExpr(S.Context, VD, false, T, VK_LValue, Loc);
   ExprResult Result = S.PerformMoveOrCopyInitialization(
       InitializedEntity::InitializeBlock(Loc, T, false), VD, VD->getType(),
       VarRef, /*AllowNRVO=*/true);