[sanitizer] Define UINTPTR_MAX

Summary:
`sanitizer_internal_defs.h` didn't have this define, which will be useful in
an upcoming CL.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D47270

llvm-svn: 333109