[LoopUnroll] Introduce PragmaUnrollFullMaxIterations as a hard cap on how many...

[LoopUnroll] Introduce PragmaUnrollFullMaxIterations as a hard cap on how many iterations we try to unroll (#78648)

Fixes [PR77842](https://github.com/llvm/llvm-project/issues/77842

) where
UBSAN causes pragma full unroll to try and unroll INT_MAX times. This
sets a cap to make sure we don't attempt this and crash the compiler.

Testing:
ninja check-all with new test

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Co-authored-by: Nikita Popov <github@npopov.com>