[iwyu] Move <cmath> out of llvm/Support/MathExtras.h

[iwyu] Move <cmath> out of llvm/Support/MathExtras.h

Interestingly, MathExtras.h doesn't use <cmath> declaration, so move it out of
that header and include it when needed.

No functional change intended, but there's no longer a transitive include
fromMathExtras.h to cmath.