[GWP-ASan] Move random-related code in the allocator

We need to have all thread specific data packed into a single `uintptr_t`
for the upcoming Fuchsia support. We can move the `RandomState` into the
`ThreadLocalPackedVariables`, reducing the size of `NextSampleCounter`
to 31 bits (or we could reduce `RandomState` to 31 bits).

We move `getRandomUnsigned32` into the platform agnostic part of the
class, and `initPRNG` in the platform specific part.

`ScopedBoolean` is replaced by actual assignments since non-const
references to bitfields are prohibited.

`random.{h,cpp}` are removed.

Differential Revision: https://reviews.llvm.org/D89908