[flang] Handle CALL C_F_POINTER(without SHAPE=)

Calls to C_F_POINTER() without the optional SHAPE= third argument
were failing to be recognized as proper calls to the intrinsic,
but the failure was not generating any error message.  This led to
a crash in lowering, which rightfully expects a typed expression
to be associated with the call.

So (1) catch silent failures to convert CALL statements as internal
errors, as is done for expressions and assignment statements; and
(2) clean up C_F_POINTER intrinsic handling to cope with only two
arguments and to emit an error for a FPTR= argument with no type.

Differential Revision: https://reviews.llvm.org/D119847