[lldb] Remove SetCount/ClearCount from Flags

Summary:
These functions are only used in tests where we should test the actual flag values instead of counting all bits for an approximate check.
Also these popcount implementation aren't very efficient and doesn't seem to be optimised to anything fast.

Reviewers: davide, JDevlieghere

Reviewed By: davide, JDevlieghere

Subscribers: abidh, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67540

llvm-svn: 372018