Remove non-standard nothrow specializations

[minchopaskal]

Fix for #7098
The specialisations of is_nothrow_constructible in StringRef.h are non-standard and as seen in the issue fail with newer versions of libc++.
Manage to compile on MacOS 26.4(clang-2100.0.123.102). The static checks in DiagnosticIDs.cpp remain though so if there is an issue the builds should show it.

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[damyanp]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[damyanp]

It looks like the code is removed in this PR was very deliberately added. Before we can accept this, we'd need to understand that removing it doesn't regress the scenario it was intended to fix.