[Coverage] Explicitly mark the l.h.s of && and || (fixes PR33465)

This makes it possible to view sub-line region counts for the l.h.s of
&& and || expressions in coverage reports.

It also fixes PR33465, which shows an example of incorrect coverage
output for an assignment statement containing '||'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315979 91177308-0d34-0410-b5e6-96231b3b80d8