[ItaniumDemangle] Unconditionally parse substitution template arguments

With the current mangled name parser, when we try to parse a type name
inside the template argument of an operator, we end up not parsing any
template arguments of substitutions, which ends up causing things to
fail in certain cases. This is compliant with the Itanium ABI
documentation as far as I can tell.

I tried to look through the git history to find out why the limitation
on parsing template arguments for substitutions inside of operators
existed, but I only ended up at the import commit, so I suspect the
original motivation is lost to time.

A regression test from LLVM has been added.

Author: boomanaiden154

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -4626,10 +4626,10 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
       //   <template-template-param> ::= <template-param>
       //                             ::= <substitution>
       //
-      // If this is followed by some <template-args>, and we're permitted to
-      // parse them, take the second production.
+      // If this is followed by some <template-args>, take the second
+      // production.
 
-      if (look() == 'I' && (!IsSubst || TryToParseTemplateArgs)) {
+      if (look() == 'I') {
         if (!IsSubst)
           Subs.push_back(Result);
         Node *TA = getDerived().parseTemplateArgs();

libcxxabi/test/test_demangle.pass.cpp

@@ -30247,6 +30247,7 @@ const char* cases[][2] = {
     {"_Z1fDSDRm", "f(_Sat unsigned long _Fract)"},
 
     {"_Z11bfloat16addDF16bDF16b", "bfloat16add(std::bfloat16_t, std::bfloat16_t)"},
+    {"_ZNK4llvm11SmallStringILj16EEcvNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEEv", "llvm::SmallString<16u>::operator std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>() const"},
     // clang-format on
 };
 

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -4626,10 +4626,10 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
       //   <template-template-param> ::= <template-param>
       //                             ::= <substitution>
       //
-      // If this is followed by some <template-args>, and we're permitted to
-      // parse them, take the second production.
+      // If this is followed by some <template-args>, take the second
+      // production.
 
-      if (look() == 'I' && (!IsSubst || TryToParseTemplateArgs)) {
+      if (look() == 'I') {
         if (!IsSubst)
           Subs.push_back(Result);
         Node *TA = getDerived().parseTemplateArgs();