[ASan][libc++] Initialize `__r_` variable with lambda (#77394)

This commit is a refactor (increases readability) and optimization fix.

This is a fixed commit of
https://github.com/llvm/llvm-project/pull/76200 First reverthed here:
https://github.com/llvm/llvm-project/commit/1ea7a56057492d9da1124787a9855cc2edca7df9

Please, check original PR for details.

The difference is a return type of the lambda.

Original description:

This commit addresses optimization and instrumentation challenges
encountered within comma constructors.
1) _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS does not work in comma
constructors.
2) Code inside comma constructors is not always correctly optimized.
Problematic code examples:
- `: __r_(((__str.__is_long() ? 0 : (__str.__annotate_delete(), 0)),
std::move(__str.__r_))) {`
- `: __r_(__r_([&](){ if(!__s.__is_long()) __s.__annotate_delete();
return std::move(__s.__r_);}())) {`

However, lambda with argument seems to be correctly optimized. This
patch uses that fact.

Use of lambda based on idea from @ldionne.