[Bug middle-end/89543] Don't modify TREE_NO_WARNING during folding in place

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89543
Bug 89543 depends on bug 89503, which changed state.

Bug 89503 Summary: Checking ICE in 'gcc.dg/warn-strlen-no-nul.c'
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89503

           What    |Removed                     |Added
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             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED