Traits.h: Fix feature-test macro for P1144 trivial relocatability

[Quuxplusone]

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Well, this is awkward. #2216 added this #if check, but with the P1144 macro typo'ed: __cpp_lib_is_trivially_relocatable should have been __cpp_lib_trivially_relocatable.

In #2216 I even pointed to both the proposal and another repo's (correct) example, and yet somehow I still typo'ed it in the actual PR, and didn't notice for months! Really sorry about this.

Compiler Explorer (left-hand pane: as it is today; right-hand pane: with the wrong macro manually defined so that you get the codegen you were supposed to be getting all along) https://godbolt.org/z/7oP3Yz61d

Attn: @Orvid, @ot