tsan: introduce Tid and StackID typedefs

Currently we inconsistently use u32 and int for thread ids,
there are also "unique tid" and "os tid" and just lots of other
things identified by integers.
Additionally new tsan runtime will introduce yet another
thread identifier that is very different from current tids.
Similarly for stack IDs, it's easy to confuse u32 with other
integer identifiers. And when a function accepts u32 or a struct
contains u32 field, it's not always clear what it is.

Add Tid and StackID typedefs to make it clear what is what.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D107152