[Demangle] Add support for D function-local parent symbols

[ljmf00]

Summary

Internally `__Sddd` function-local parent symbols are used to solve ambiguities on symbols in
the same scope with the same mangled name.

[dblaikie]

Looks good!

llvm/lib/Demangle/DLangDemangle.cpp

const char *Demangler::parseIdentifier(OutputBuffer *Demangled,
 
   if (strlen(Endptr) < Len)
     return nullptr;
 
   Mangled = Endptr;
 
   // TODO: Parse template instances with a length prefix.
 
+  // There can be multiple different declarations in the same function that
+  // have the same mangled name.  To make the mangled names unique, a fake
+  // parent in the form `__Sddd' is added to the symbol.
+  if (Len >= 4 && Mangled[0] == '_' && Mangled[1] == '_' && Mangled[2] == 'S') {
+    const char *NumPtr = Mangled + 3;
+    while (NumPtr < (Mangled + Len) && std::isdigit(*NumPtr))
+      ++NumPtr;
+
+    if (Mangled + Len == NumPtr) {
+      // Skip over the fake parent.
+      Mangled += Len;
+      return parseIdentifier(Demangled, Mangled);
+    }
+
+    // Else demangle it as a plain identifier.
+  }
+
   return parseLName(Demangled, Mangled, Len);
 }
 
 const char *Demangler::parseLName(OutputBuffer *Demangled, const char *Mangled,
                                   unsigned long Len) {
   switch (Len) {
   case 6:
     if (strncmp(Mangled, "__initZ", Len + 1) == 0) {

llvm/unittests/Demangle/DLangDemangleTest.cpp

testing::Values(
         std::make_pair("_D8demangle4test6__initZ",
                        "initializer for demangle.test"),
         std::make_pair("_D8demangle4test6__vtblZ", "vtable for demangle.test"),
         std::make_pair("_D8demangle4test7__ClassZ",
                        "ClassInfo for demangle.test"),
         std::make_pair("_D8demangle4test11__InterfaceZ",
                        "Interface for demangle.test"),
         std::make_pair("_D8demangle4test12__ModuleInfoZ",
-                       "ModuleInfo for demangle.test")));
+                       "ModuleInfo for demangle.test"),
+        std::make_pair("_D8demangle4__S14testZ", "demangle.test"),
+        std::make_pair("_D8demangle4__Sd4testZ", "demangle.__Sd.test")));