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b964419ec2
Previously, libcxx forced all strings created during constant evaluation to point to allocated memory. That was done due to implementation difficultites, but it turns out not to be necessary. This patch permits the use of SSO strings during constant evaluation, and also simplifies the implementation. This does have a downside in terms of enabling users to accidentally write non-portable code, however, which I've documented in UsingLibcxx.rst. In particular, whether `constinit std::string x = "...";` will successfully compile now depends on whether the string is smaller than the SSO capacity -- in libc++, up to 22 bytes on 64-bit platforms, and up to 10 bytes on 32-bit platforms. By comparison, libstdc++ and MSVC have an SSO capacity of 15 bytes, except that in libstdc++, constant-initialized strings cannot be used as function-locals because the object contains a pointer to itself. Closes #68434