Fix Sema for compares with _Atomic vars.

Use UsualArithmeticConversions unconditionally in analysis of
comparisons and conditional operators: the method performs
the usual arithmetic conversions if both sides are arithmetic, and
usual unary conversions if they are not.  This is just a cleanup
for conditional operators; for comparisons, it fixes the issue that
we would try to check isArithmetic() on an atomic type.

Also, fix GetExprRange() in SemaChecking.cpp so it deals with variables
of atomic type correctly.

Fixes PR15537.