Fix crash in EmitDeclMetadata mode

[loladiro]

Summary

This fixes a bug that's easily encountered in LLDB (https://llvm.org/bugs/show_bug.cgi?id=22875). The problem here is that we mangle a name during debug info emission, but never actually emit the actual Decl, so we run into problems here. I am not entirely sure this is the right fix. The comment says something about StaticLocalDeclMap, which may be better here, but I checked and that already existed when this functionality was added, which makes me think that there is some reason it wasn't used.

[labath]

I'll just observe this one. Someone with more knowledge of clang needs to review this.

[rjmccall]

LGTM.

[loladiro]

Can you provide insight as to why this is looking at MangledDeclNames rather than the StaticLocalDeclMap? I'd like to extend the comment with an explanation.

cfe/trunk/lib/CodeGen/CodeGenModule.cpp

 /// projects using IR gen as a subroutine.
 ///
 /// Since there's currently no way to associate an MDNode directly
 /// with an llvm::GlobalValue, we create a global named metadata
 /// with the name 'clang.global.decl.ptrs'.
 void CodeGenModule::EmitDeclMetadata() {
   llvm::NamedMDNode *GlobalMetadata = nullptr;
 
-  // StaticLocalDeclMap
   for (auto &I : MangledDeclNames) {
     llvm::GlobalValue *Addr = getModule().getNamedValue(I.second);
+    // Some mangled names don't necessarily have an associated GlobalValue
+    // in this module, e.g. if we mangled it for DebugInfo.
+    if (Addr)
       EmitGlobalDeclMetadata(*this, GlobalMetadata, I.first, Addr);
   }
 }
 
 /// Emits metadata nodes for all the local variables in the current
 /// function.
 void CodeGenFunction::EmitDeclMetadata() {
   if (LocalDeclMap.empty()) return;