[Demangle] Support Rust v0 mangling scheme in llvm::demangle

[tmiasko]

Summary

The llvm::demangle is currently used by llvm-objdump and llvm-readobj,
so this effectively adds support for Rust v0 mangling to those
applications.

[dblaikie]

Sounds good

llvm/lib/Demangle/Demangle.cpp

 #include <cstdlib>
 
 static bool isItaniumEncoding(const std::string &MangledName) {
   size_t Pos = MangledName.find_first_not_of('_');
   // A valid Itanium encoding requires 1-4 leading underscores, followed by 'Z'.
   return Pos > 0 && Pos <= 4 && MangledName[Pos] == 'Z';
 }
 
+static bool isRustEncoding(const std::string &MangledName) {
+  return MangledName.size() >= 2 && MangledName[0] == '_' &&
+         MangledName[1] == 'R';
+}
+
 std::string llvm::demangle(const std::string &MangledName) {
   char *Demangled;
   if (isItaniumEncoding(MangledName))
     Demangled = itaniumDemangle(MangledName.c_str(), nullptr, nullptr, nullptr);
+  else if (isRustEncoding(MangledName))
+    Demangled = rustDemangle(MangledName.c_str(), nullptr, nullptr, nullptr);
   else
     Demangled = microsoftDemangle(MangledName.c_str(), nullptr, nullptr,
                                   nullptr, nullptr);
 
   if (!Demangled)
     return MangledName;
 
   std::string Ret = Demangled;
   std::free(Demangled);
   return Ret;
 }

llvm/unittests/Demangle/DemangleTest.cpp

TEST(Demangle, demangleTest) {
   EXPECT_EQ(demangle("_Z3fooi"), "foo(int)");
   EXPECT_EQ(demangle("__Z3fooi"), "foo(int)");
   EXPECT_EQ(demangle("___Z3fooi_block_invoke"),
             "invocation function for block in foo(int)");
   EXPECT_EQ(demangle("____Z3fooi_block_invoke"),
             "invocation function for block in foo(int)");
   EXPECT_EQ(demangle("?foo@@YAXH@Z"), "void __cdecl foo(int)");
   EXPECT_EQ(demangle("foo"), "foo");
+  EXPECT_EQ(demangle("_RNvC3foo3bar"), "foo::bar");
 
   // Regression test for demangling of optional template-args for vendor
   // extended type qualifier (https://bugs.llvm.org/show_bug.cgi?id=48009)
   EXPECT_EQ(demangle("_Z3fooILi79EEbU7_ExtIntIXT_EEi"),
             "bool foo<79>(int _ExtInt<79>)");
 }