[clang][CodeGen][UBSan] VLA size checking for unsigned integer parameter

The code generation for the UBSan VLA size check was qualified by a con-
dition that the parameter must be a signed integer, however the C spec
does not make any distinction that only signed integer parameters can be
used to declare a VLA, only qualifying that it must be greater than zero
if it is not a constant.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D116048