[llvm][Z3][NFC] Improve mkBitvector performance

We convert `APSInt`s to Z3 Bitvectors in an inefficient way for most cases.
We should not serialize to std::string just to pass an int64 integer.

For the vast majority of cases, we use at most 64-bit width integers (at least
in the Clang Static Analyzer). We should simply call the `Z3_mk_unsigned_int64`
and `Z3_mk_int64` instead of the `Z3_mk_numeral` as stated in the Z3 docs.
Which says:
> It (`Z3_mk_unsigned_int64`, etc.) is slightly faster than `Z3_mk_numeral` since
> it is not necessary to parse a string.

If the `APSInt` is wider than 64 bits, we will use the `Z3_mk_numeral` with a
`SmallString` instead of a heap-allocated `std::string`.

Differential Revision: https://reviews.llvm.org/D78453