Broaden -Wstring-conversion to catch string literals in logical or expressions.
Previously, string literals were ignored in all logical expressions.  This
reduces it to only ignore in logical and expressions.

assert(0 && "error"); // No warning
assert(0 || "error"); // Warn

Fixes PR17565


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200056 91177308-0d34-0410-b5e6-96231b3b80d8