We just want to test whether the language switch works.
This is easier to control for libc++, since for bots building
the tests against libstdc++ we might not have the necessary
`<compare>` header available currently.
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.
