Push down more common code into PlatformPOSIX

Summary:
- GetFileWithUUID: All platforms except PlatformDarwin had this.
However, I see no reason why this code would not apply there as well.

- GetProcessInfo, FindProcesses: The implementation was the same in all classes.

- GetFullNameForDylib: This code should apply to all non-darwin
platforms. I've kept the PlatformDarwin override as the situation is
different there.

Reviewers: clayborg, krytarowski, emaste

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D29496

llvm-svn: 294019