[lldb/Dataformatters] Adapt C++ std::string dataformatter for D128285

This patch changes the C++ `std::string` dataformatter to reflect
internal layout changes following D128285.

Now, in short-mode strings, in order to access the `__size_` and
`__is_long_` attributes, we need to access a packed anonymous struct,
which introduces another indirection.

We need to do the same in order to access the `__cap_` field for
long-mode strings.

This should fix the various test failures that are happening on
GreenDragon:

https://green.lab.llvm.org/green/job/lldb-cmake/44918/

rdar://96010248

Differential Revision: https://reviews.llvm.org/D128694



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>