Issue local statics in correct DWARF lexical scope

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Summary

This is a matching change to an LLVM patch to ensure local statics are issued to the DWARF in the correct lexical scope.

This change detects the static being in a lexical block and ensures the debug into meta data is assigned to that scope.

A matching test will be added to debuginfo-tests.

[probinson]

+cfe-commits
See also the LLVM change in D42369 and a debuginfo-tests test in D42371.

[JDevlieghere]

This makes sense to me, LGTM.

[rnk]

Still needs a test.

[dblaikie]

Just to clarify - the general philosophy (there are many cases of exceptions due to complications in various areas) is that clang changes should have clang tests (source code to IR), LLVM changes should have LLVM tests (IR to assembly or object code+dwarf dump) & if debuginfo-tests are optional/as-desired.

lib/CodeGen/CGDebugInfo.cpp

void CGDebugInfo::collectVarDeclProps(const VarDecl *VD, llvm::DIFile *&Unit,
   // member, and the record type is marked as __declspec(dllexport), an implicit
   // definition of the member will be created in the record context.  DWARF
   // doesn't seem to have a nice way to describe this in a form that consumers
   // are likely to understand, so fake the "normal" situation of a definition
   // outside the class by putting it in the global scope.
   if (DC->isRecord())
     DC = CGM.getContext().getTranslationUnitDecl();
 
+  // If we are in a lexical block use it as the context.
+  if (!LexicalBlockStack.empty()) 
+    VDContext = cast<llvm::DIScope>(LexicalBlockStack.back());
+  else {
     llvm::DIScope *Mod = getParentModuleOrNull(VD);
     VDContext = getContextDescriptor(cast<Decl>(DC), Mod ? Mod : TheCU);
   }
+}
 
 llvm::DISubprogram *CGDebugInfo::getFunctionFwdDeclOrStub(GlobalDecl GD,
                                                           bool Stub) {
   llvm::DINodeArray TParamsArray;
   StringRef Name, LinkageName;
   llvm::DINode::DIFlags Flags = llvm::DINode::FlagZero;
   SourceLocation Loc = GD.getDecl()->getLocation();
   llvm::DIFile *Unit = getOrCreateFile(Loc);