[compiler-rt][NFC] Avoid implicit-integer-sign-change in FuzzedDataProvider::ConsumeIntegralInRange

This makes the implicit conversion that is happening explicit.
Otherwise, each user is forced to suppress this
implicit-integer-sign-change runtime error in their their UBSAN
suppressions file.

For example, the runtime error might look like:

runtime error: implicit conversion from type 'long' of value -9223372036854775808 (64-bit, signed) to type 'uint64_t' (aka 'unsigned long') changed the value to 9223372036854775808 (64-bit, unsigned)
    #0 0x55fe29dea91d in long FuzzedDataProvider::ConsumeIntegralInRange<long>(long, long) src/./test/fuzz/FuzzedDataProvider.h:233:25
    [...]
SUMMARY: UndefinedBehaviorSanitizer: implicit-integer-sign-change test/fuzz/FuzzedDataProvider.h:233:25 in

Differential Revision: https://reviews.llvm.org/D155206