[simd.overview] Remove spurious noexcept

[hewillk]

This function is defined without noexcept, which is reasonable since gen can throw.
Is this the editorial or the LWG?

[eisenwave]

This is a wording bug in cplusplus/papers#2097

The paper removes noexcept from those constructors, but makes no changes to the synopsis, and neither LWG nor editors noticed that. The intent is clearly to remove noexcept from the constructors of basic_vec and basic_mask, so this issue is editorial.

We should make corresponding changes to basic_mask, which also has a stray noexcept in the synopsis.

[eisenwave]

Gotta fix the message on the new commit.