Add ABSL_ATTRIBUTE_LIFETIME_BOUND to Flag help generator

Since this method is only called by `NonConst` which immediately converts it to a `std::string`, this is currently safe. Add annotation nevertheless to show the contract.

This currently makes the majority of lifetime annotation suggestion provided by the lifetime analysis https://godbolt.org/z/hKvrE1hG1

PiperOrigin-RevId: 843275432
Change-Id: Ib1a0513eca944a9c7c4c612c3111bf05881c746d

Author: Abseil Team

absl/flags/flag.h

@@ -241,7 +241,8 @@ ABSL_NAMESPACE_END
     /* default value argument. That keeps temporaries alive */               \
     /* long enough for NonConst to work correctly.          */               \
     static constexpr absl::string_view Value(                                \
-        absl::string_view absl_flag_help = ABSL_FLAG_IMPL_FLAGHELP(txt)) {   \
+        absl::string_view absl_flag_help ABSL_ATTRIBUTE_LIFETIME_BOUND  =     \
+            ABSL_FLAG_IMPL_FLAGHELP(txt)) {                                  \
       return absl_flag_help;                                                 \
     }                                                                        \
     static std::string NonConst() { return std::string(Value()); }           \