[analyzer] Always use 'bool' as the SValBuilder condition type in C++.

We have assertions for this, but a few edge cases had snuck through where
we were still unconditionally using 'int'.

<rdar://problem/15703011>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197733 91177308-0d34-0410-b5e6-96231b3b80d8