Instead of re-implementing `Type::isIntegralType`, call it explicitly.
This means we get support for `BitIntType` out-of-the-box.
We don't use `IsIntegerType` here because we want to abide by the
language-specific notions of an integer type (which differ between C++
and C).
The slight behaviour change here is that `IsIntegerType` will now treat
complete enumerations as integers in C. This is correct according to the
C standard.
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