Implement demangling support for C++20 lambda expression extensions.

This implements demangling support for the mangling extensions specified
in itanium-cxx-abi/cxx-abi#85, much of which is
implemented in Clang r359967 and r371004.

Specifically, this provides demangling for:

 * <template-param-decl> in <lambda-sig>
 * <template-param> with non-zero level
 * lambda-expression literals (not emitted by Clang yet)
 * nullptr literals
 * string literals

(The final two seem unrelated, but handling them was necessary in order
to disambiguate between lambda expressions and the other forms of
literal for which we have a type but no value.)

When demangling a <lambda-sig>, we form template parameters with no
corresponding argument, so we cannot substitute in the argument in the
demangling. Instead we invent synthetic names for the template
parameters (eg, '[]<typename $T>($T *x)').

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@371273 91177308-0d34-0410-b5e6-96231b3b80d8

Author: zygoloid

src/cxa_demangle.cpp

@@ -171,6 +171,16 @@ struct DumpVisitor {
       return printStr("SpecialSubKind::iostream");
     }
   }
+  void print(TemplateParamKind TPK) {
+    switch (TPK) {
+    case TemplateParamKind::Type:
+      return printStr("TemplateParamKind::Type");
+    case TemplateParamKind::NonType:
+      return printStr("TemplateParamKind::NonType");
+    case TemplateParamKind::Template:
+      return printStr("TemplateParamKind::Template");
+    }
+  }
 
   void newLine() {
     printStr("\n");