[demangler] Ignore leading underscores if present

On MacOS, symbols start with a leading underscore, so just parse and
ignore it if present.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351481 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/llvm/Demangle/ItaniumDemangle.h b/include/llvm/Demangle/ItaniumDemangle.h
index 53107c9..9a54ff5 100644
--- a/include/llvm/Demangle/ItaniumDemangle.h
+++ b/include/llvm/Demangle/ItaniumDemangle.h
@@ -5143,7 +5143,7 @@
 // extension      ::= ___Z <encoding> _block_invoke_<decimal-digit>+
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parse() {
-  if (consumeIf("_Z")) {
+  if (consumeIf("_Z") || consumeIf("__Z")) {
     Node *Encoding = getDerived().parseEncoding();
     if (Encoding == nullptr)
       return nullptr;
@@ -5156,7 +5156,7 @@
     return Encoding;
   }
 
-  if (consumeIf("___Z")) {
+  if (consumeIf("___Z") || consumeIf("____Z")) {
     Node *Encoding = getDerived().parseEncoding();
     if (Encoding == nullptr || !consumeIf("_block_invoke"))
       return nullptr;