Teach -Wuninitialized about C++'s typeid expression, including both the

Teach -Wuninitialized about C++'s typeid expression, including both the
evaluated and unevaluated contexts. Add some testing of sizeof and
typeid.

Both of the typeid tests added here were triggering warnings previously.
Now the one false positive is suppressed without suppressing the warning
on actually buggy code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@129431 91177308-0d34-0410-b5e6-96231b3b80d8