[ItaniumDemangle] Add template name to the substitutions list during demangling (#108538)

When demangling a template template parameter (`method<bool,
Bar>(Bar<bool> b)`), the current demangler version first enters the
template argument (`bool`) into the substitutions list, then the whole
template specialization (`Bar<bool>`). The template name (`Bar`) never
becomes a substitution candidate on its own.

This is different when mangling. Mangling `method<bool, Bar>(Bar<bool>
b, Bar<int> i)` substitutes the `Bar` in the second parameter with the
substitution for `TemplateTemplateParmDecl`.

This leads to a discrepancy between mangler and demangler, see
#108009.

Author: VitaNuo

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -4450,6 +4450,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
     // parse them, take the second production.
 
     if (TryToParseTemplateArgs && look() == 'I') {
+      Subs.push_back(Result);
       Node *TA = getDerived().parseTemplateArgs();
       if (TA == nullptr)
         return nullptr;

libcxxabi/test/test_demangle.pass.cpp

@@ -30024,6 +30024,9 @@ const char* cases[][2] =
     // See https://github.com/itanium-cxx-abi/cxx-abi/issues/165.
     {"_ZN1C1fIiEEvDTtlNS_UlT_TL0__E_EEE", "void C::f<int>(decltype(C::'lambda'(int, auto){}))"},
 
+    // See https://github.com/llvm/llvm-project/issues/108009.
+    {"_ZN3FooIiE6methodIb3BarEEvT0_IT_ES3_IiE", "void Foo<int>::method<bool, Bar>(Bar<bool>, Bar<int>)"},
+
     // C++20 class type non-type template parameters:
     {"_Z1fIXtl1BLPi0ELi1EEEEvv", "void f<B{(int*)0, 1}>()"},
     {"_Z1fIXtl1BLPi32EEEEvv", "void f<B{(int*)32}>()"},

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -4450,6 +4450,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
     // parse them, take the second production.
 
     if (TryToParseTemplateArgs && look() == 'I') {
+      Subs.push_back(Result);
       Node *TA = getDerived().parseTemplateArgs();
       if (TA == nullptr)
         return nullptr;