[RFC/WIP][clang] Fix printing of names of inherited constructors

[labath]

Summary

This is a fairly hacky fix to the following problem: Debug information
entries for inherited constructors are emitted with the name of the base
class, instead of the inheriting class.

This problem can be tracked down to a FIXME in
Sema::findInheritingConstructor, which works around the problem of not
having a DeclarationName to describe inherited constructors. The current
patch compounds that workaround by printing the inherited constructor
name in a different way.

I don't really expect this patch to make its way into the tree in the
current form. I am mainly putting it up to point out the problem and
start a discussion on the solution. I suppose the right fix is to
implement the FIXME, and add a new DeclarationName kind. But that seems
like it could be a fairly involved change, so I wanted to get some
confirmation (and maybe a bit of guidance) first.

[dblaikie]

@rsmith - this changes printing the name of inherited constructors in general, not just for debug info - could you take a gander and make sure that's probably OK for other callers trying to print the name of an inheriting constructor?

clang/include/clang/AST/DeclCXX.h

public:
 
   CXXConstructorDecl *getCanonicalDecl() override {
     return cast<CXXConstructorDecl>(FunctionDecl::getCanonicalDecl());
   }
   const CXXConstructorDecl *getCanonicalDecl() const {
     return const_cast<CXXConstructorDecl*>(this)->getCanonicalDecl();
   }
 
+  void printName(raw_ostream &os) const override {
+    if (isInheritingConstructor())
+      getParent()->printName(os);
+    else
+      CXXMethodDecl::printName(os);
+  }
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == CXXConstructor; }
 };
 
 /// Represents a C++ destructor within a class.
 ///
 /// For example:

clang/test/CodeGenCXX/debug-info-inheriting-constructor.cpp

 // CHECK: define void @{{.*}}foo
 // CHECK-NOT: ret void
 // CHECK: call void @llvm.dbg.declare
 // CHECK-NOT: ret void
 // CHECK: call void @llvm.dbg.declare(metadata %{{.*}}** %{{[^,]+}},
 // CHECK-SAME: metadata ![[THIS:[0-9]+]], metadata !DIExpression()), !dbg ![[LOC:[0-9]+]]
 // CHECK: ret void, !dbg ![[NOINL:[0-9]+]]
 // CHECK: ![[FOO:.*]] = distinct !DISubprogram(name: "foo"
-// CHECK-DAG: ![[A:.*]] = distinct !DISubprogram(name: "A", linkageName: "_ZN1BCI11AEiz"
+// CHECK-DAG: ![[B:.*]] = distinct !DISubprogram(name: "B", linkageName: "_ZN1BCI11AEiz"
 void foo() {
-// CHECK-DAG: ![[LOC]] = !DILocation(line: 0, scope: ![[A]], inlinedAt: ![[INL:[0-9]+]])
+  // CHECK-DAG: ![[LOC]] = !DILocation(line: 0, scope: ![[B]], inlinedAt: ![[INL:[0-9]+]])
   // CHECK-DAG: ![[INL]] = !DILocation(line: [[@LINE+1]], scope: ![[FOO]])
   B b(0);
 // CHECK: ![[NOINL]] = !DILocation(line: [[@LINE+1]], scope: !{{[0-9]+}})
 }

clang/test/CodeGenCXX/debug-info-inlined.cpp

 
 // CHECK: define{{.*}}A
 // CHECK-NOT: {{ ret }}
 // CHECK: store %class.Forward* %
 // CHECK-SAME: %class.Forward** %
 // CHECK-SAME: !dbg ![[INL:[0-9]+]]
 
 // CHECK: ![[INL]] = !DILocation(line: 10, scope: ![[SP:[0-9]+]], inlinedAt:
-// CHECK: ![[SP]] = distinct !DISubprogram(name: "Base", {{.*}} DISPFlagDefinition
+// CHECK: ![[SP]] = distinct !DISubprogram(name: "Forward", {{.*}} DISPFlagDefinition