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

This implements demangling support for the mangling extensions specified
in https://github.com/itanium-cxx-abi/cxx-abi/pull/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)').

llvm-svn: 371273

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