IDL compiler: Use C++ type for local variable in [Clamp]

In current compiler, [Clamp] uses a local variable with the IDL type
(|unsigned short| or |unsigned long|), instead of the C++ type (in both
these cases |unsigned|).
This is then implicitly converted to |unsigned| on use.
It's clearer (and likely safer and more correct) to use the internal C++
type from the start, which this CL does.

BUG=239771

Review URL: https://codereview.chromium.org/54033002

git-svn-id: svn://svn.chromium.org/blink/trunk@161035 bbb929c8-8fbe-4397-9dbb-9b2b20218538