[clang-tidy] Warn on functional C-style casts

The google-readability-casting check is meant to be on par
with cpplint's readability/casting check, according to the
documentation. However it currently does not diagnose
functional casts, like:

float x = 1.5F;
int y = int(x);

This is detected by cpplint, however, and the guidelines
are clear that such a cast is only allowed when the type
is a class type (constructor call):

> You may use cast formats like `T(x)` only when `T` is a class type.

Therefore, update the clang-tidy check to check this
case.

Differential Revision: https://reviews.llvm.org/D114427