[analyzer] Extend IdenticalExprChecker to check ternary operator results.

Warn if both result expressions of a ternary operator (? :) are the same.
Because only one of them will be executed, this warning will fire even if
the expressions have side effects.

Patch by Anders Rönnholm and Per Viberg!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196937 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Misc. tidying in IdenticalExprChecker.

Some things I missed when this first went in.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196938 91177308-0d34-0410-b5e6-96231b3b80d8