[DebugInfo] Add an attribute to force type info to be emitted for

class types.

The goal is to provide a way to bypass constructor homing when emitting
class definitions and force class definitions in the debug info.

Not sure about the wording of the attribute, or whether it should be
specific to classes with constructors

clang/include/clang/Basic/Attr.td

@@ -1660,6 +1660,13 @@ def NoDebug : InheritableAttr {
   let Documentation = [NoDebugDocs];
 }
 
+def DebugTypeInfoAsNeeded : InheritableAttr {
+  let Spellings = [Clang<"debug_type_info_as_needed">];
+  let Subjects = SubjectList<[CXXRecord]>;
+  let Documentation = [Undocumented];
+  let SimpleHandler = 1;
+}
+
 def NoDuplicate : InheritableAttr {
   let Spellings = [Clang<"noduplicate">];
   let Subjects = SubjectList<[Function]>;

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -2344,6 +2344,10 @@ static bool shouldOmitDefinition(codegenoptions::DebugInfoKind DebugKind,
   if (!CXXDecl)
     return false;
 
+  // Don't omit definition if marked with attribute.
+  if (RD->hasAttr<DebugTypeInfoAsNeededAttr>())
+    return false;
+
   // Only emit complete debug info for a dynamic class when its vtable is
   // emitted.  However, Microsoft debuggers don't resolve type information
   // across DLL boundaries, so skip this optimization if the class or any of its

clang/test/CodeGenCXX/debug-info-emit-as-needed-attribute.cpp

@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -DSETATTR=0 -emit-llvm -std=c++14 -debug-info-kind=constructor %s -o - | FileCheck %s --check-prefix=DEBUG
+// RUN: %clang_cc1 -DSETATTR=1 -emit-llvm -std=c++14 -debug-info-kind=constructor %s -o - | FileCheck %s --check-prefix=WITHATTR
+
+#if SETATTR
+#define DEBUGASNEEDED __attribute__((debug_type_info_as_needed))
+#else
+#define DEBUGASNEEDED
+#endif
+
+// Struct that isn't constructed, so its full type info should be omitted with
+// -debug-info-kind=constructor.
+struct DEBUGASNEEDED some_struct {
+  some_struct() {}
+};
+
+void func1(some_struct s) {}
+// void func2() { func1(); }
+// DEBUG:  !DICompositeType({{.*}}name: "some_struct"
+// DEBUG-SAME:              flags: {{.*}}DIFlagFwdDecl
+// WITHATTR: !DICompositeType({{.*}}name: "some_struct"
+// WITHATTR-NOT: DIFlagFwdDecl
+