Properly consider the range of enum for range comparisons in C mode

In some cases, clang applies the C++ rules for computing the range of a
value when said value is an enum.

Instead, apply C semantics when in C mode.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183084 91177308-0d34-0410-b5e6-96231b3b80d8