[FileCheck] Fix naming of OverflowErrorStr var

As pointed out by Joel E. Denny in D97845, the OverflowErrorStr variable
is misnamed because the error is raised for any parsing error. Note that
in FileCheck proper this only happens in case of (under|over)flow
because the regex will ensure a number in the correct format is matched.

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D98342