[lldb] Clean up uses of UuidCompatibility.h

This commit does a few related things:
- Removes unused function `uuid_is_null`
- Removes unneeded includes of UuidCompatibility.h
- Renames UuidCompatibility to AppleUuidCompatibility and adds a comment
  to clarify intent of header.
- Moves AppleUuidCompatibility to the include directory

Differential Revision: https://reviews.llvm.org/D156562