[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/CodeGen/CGOpenMPRuntime.cpp b/clang/lib/CodeGen/CGOpenMPRuntime.cpp
index d7a0a72..746d3d3 100644
--- a/clang/lib/CodeGen/CGOpenMPRuntime.cpp
+++ b/clang/lib/CodeGen/CGOpenMPRuntime.cpp
@@ -353,7 +353,7 @@
       if (VD->isLocalVarDeclOrParm())
         continue;
 
-      DeclRefExpr DRE(const_cast<VarDecl *>(VD),
+      DeclRefExpr DRE(CGF.getContext(), const_cast<VarDecl *>(VD),
                       /*RefersToEnclosingVariableOrCapture=*/false,
                       VD->getType().getNonReferenceType(), VK_LValue,
                       C.getLocation());