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197bd3e5e315f408bdc2f7111a0652be7f47609f by Jorg Brown <jorg@google.com>:

ConvertibleToStringView wastes a lot of cycles initializing a std::string
member that it never uses.  This CL restricts ConvertibleToStringView to just
the cases that don't nned a std::string member, and uses an additional StrSplit
overload to catch when users pass a temporary string.

This makes StrSplit() 0-30% faster depending on the use case.

PiperOrigin-RevId: 341378221
GitOrigin-RevId: 197bd3e5e315f408bdc2f7111a0652be7f47609f
Change-Id: Id863f2a3d298ed5a086ac912afab42dc30601be5