This patch allows users to evaluate expressions using
`expr -l c++20`. Currently DWARF keeps the CU's at
`DW_AT_language` at `DW_LANG_C_plus_plus_14` even
when compiling with `-std=c++20`. So even in "C++20
programs" expression evaluation will by default be
performed in `C++11` mode for now.

Enabling `C++14` has been previously attempted at
https://reviews.llvm.org/D80308

There are some remaining issues around evaluating C++20
expressions. Mainly, lack of support for C++20 AST nodes in
`clang::ASTImporter`. But these can be addressed in follow-up
patches.

credits to @jmagee

This patch removes the unused "using" declarations, updates comments,
and removes #include "llvm/ADT/Optional.h".

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to clean up the "using" declarations, #include
"llvm/ADT/Optional.h", etc.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch adds #include <optional> to those files containing
llvm::Optional<...> or Optional<...>.

I'll post a separate patch to actually replace llvm::Optional with
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

std::optional::value() has undesired exception checking semantics and is
unavailable in older Xcode (see _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS). The
call sites block std::optional migration.

These .cpp files do not use llvm::None anymore.

Since these are not header files, we can remove them pretty safely
without deprecating them first.

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch adds a way to extract the return type out
of the `CPlusPlusNameParser`. This will be useful
for cases where we want a function's basename *and* the
return type but not the function arguments; this is
currently not possible (the parser either gives us the
full name or just the basename). Since the parser knows
how to handle return types already we should just expose
this to users that need it.

**Testing**

* Added unit-tests

Differential Revision: https://reviews.llvm.org/D136935

This patch teaches the `CPlusPlusNameParser` to parse the
demangled/prettified [[gnu::abi_tag(...)]] attribute. The demangled format
isn't standardized and the additions to the parser were mainly driven
using Clang (and the limited information on this from the official
Clang docs).

This change is motivated by multiple failures around step-in
behaviour for libcxx APIs (many of which have ABI tags as of recently).
LLDB determines whether the `step-avoid-regexp` matches the current
frame by parsing the scope-qualified name out of the demangled
function symbol. On failure, the `CPlusPlusNameParser` will simply
return the fully demangled name (which can include the return type)
to the caller, which in `FrameMatchesAvoidCriteria` means we will
not correctly decide whether we should stop at a frame or not if
the function has an abi_tag.

Ideally we wouldn't want to rely on the non-standard format
of demangled attributes. Alternatives would be:

1. Use the mangle tree API to do the parsing for us
2. Reconstruct the scope-qualified name from DWARF instead of parsing
   the demangled name

(1) isn't feasible without a significant refactor of `lldb_private::Mangled`,
if we want to do this efficiently.

(2) could be feasible in cases where debug-info for a frame is
available. But it does mean we certain operations (such as step-in regexp,
and frame function names) won't work with/won't show ABI tags.

**Testing**

* Un-XFAILed step-in API test
* Added parser unit-tests

Differential Revision: https://reviews.llvm.org/D136306