[NFC][Sanitizer] Rename BufferedStackTrace::FastUnwindStack

FastUnwindStack -> UnwindFast
SlowUnwindStack -> UnwindSlow
Stack is redundant, verb should come first.

SlowUnwindStackWithContext(uptr pc, void *context, u32 max_depth) ->
SlowUnwindStack
WithContext is redundant, since it is a required parameter.

Reviewers: vitalybuka

Differential Revision: https://reviews.llvm.org/D58551

llvm-svn: 354696