Dear all,

the attached patch fixes argument checking of:

- C_SIZEOF - rejects-valid (see below) and ICE-on-valid
- C_F_POINTER - ICE-on-invalid

The interesting part is that C_SIZEOF was not well specified until
after F2018, where an interp request lead to an edit that actually
loosened restrictions and makes the checking much more straightforward,
since expressions and function results are now allowed.

I've added references to the relevant text and interp in the commit message.

While updating the checking code shared between C_SIZEOF and C_F_POINTER,
I figured that the latter missed a check preventing an ICE-on-invalid
when a function returning a pointer was passed.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald