Thread safety analysis: Rename parameters of ThreadSafetyAnalyzer::intersectAndWarn (NFC)

In D104261 we made the parameters' meaning slightly more specific, this
changes their names accordingly. In all uses we're building a new lock
set by intersecting existing locksets. The first (modifiable) argument
is the new lock set being built, the second (non-modifiable) argument is
the exit set of a preceding block.

Reviewed By: aaron.ballman, delesley

Differential Revision: https://reviews.llvm.org/D104649