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

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

This is a fixed commit of
llvm/llvm-project#76200 First reverthed here:
llvm/llvm-project@1ea7a56

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.

NOKEYCHECK=True
GitOrigin-RevId: 75efddba0f507282df479a6e296d67fd88aed489

include/string

@@ -922,7 +922,7 @@ public:
       // Turning off ASan instrumentation for variable initialization with _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS
       // does not work consistently during initialization of __r_, so we instead unpoison __str's memory manually first.
       // __str's memory needs to be unpoisoned only in the case where it's a short string.
-      : __r_(((__str.__is_long() ? 0 : (__str.__annotate_delete(), 0)), std::move(__str.__r_))) {
+      : __r_([](basic_string &__s) -> decltype(__s.__r_)&& { if(!__s.__is_long()) __s.__annotate_delete(); return std::move(__s.__r_); }(__str)) {
     __str.__r_.first() = __rep();
     __str.__annotate_new(0);
     if (!__is_long())