Fortran: fix "unstable" interfaces of external procedures [PR122206]

In the testcase repeated invocations of a function showed an apparently
unstable interface.  This was caused by trying to guess an (inappropriate)
interface of the external procedure after processing of the procedure
arguments in gfc_conv_procedure_call.  The mis-guessed interface showed up
in subsequent uses of the procedure symbol in gfc_conv_procedure_call.  The
solution is to check for an existing interface of an external procedure
before trying to wildly guess based on just the actual arguments.

	PR fortran/122206

gcc/fortran/ChangeLog:

	* trans-types.cc (gfc_get_function_type): Do not clobber an
	existing procedure interface.

gcc/testsuite/ChangeLog:

	* gfortran.dg/interface_abstract_6.f90: New test.

(cherry picked from commit c474a50b42ac3f7561f628916cf58810044986b3)