Now that xpcom strings use templates we can combine their `ArenaStrdup`
implementations. `nsTStringRepr` is used so as to allow handling of both
`nsTLiteralString` and `nsTSubstring` types.
--HG--
extra : rebase_source : e77891da589f320507b45f524a3203b3dc9f38e6
extra : histedit_source : 79d3b9a1add191563e0985f0c0e416bd29f24351
This adds support for duplicating both raw `char*` strings and raw `char16_t*`
strings by making the character type generic.
--HG--
extra : rebase_source : bb5245ed71161c8c785684e5a56ac894f03874ea
extra : histedit_source : e94eb738a3982f0cb63a894a0cdfbdf0be2b9cad