Fix bogus -Wstringop-overflow warning

Fix bogus -Wstringop-overflow warning

If you compile the testcase with -O2 -fno-inline -Wall, you get:

In function 'process_array3':
cc1: warning: 'process_array4' accessing 4 bytes in a region of size 3 [-
Wstringop-overflow=]
cc1: note: referencing argument 1 of type 'char[4]'
t.c:6:6: note: in a call to function 'process_array4'
    6 | void process_array4 (char a[4], int n)
      |      ^~~~~~~~~~~~~~
cc1: warning: 'process_array4' accessing 4 bytes in a region of size 3 [-
Wstringop-overflow=]
cc1: note: referencing argument 1 of type 'char[4]'
t.c:6:6: note: in a call to function 'process_array4'

That's because the ICF IPA pass has identified the two functions and turned
process_array3 into a wrapper of process_array4.

gcc/
	* gimple-ssa-warn-access.cc (pass_waccess::check_call): Return
	early for calls made from thunks.

gcc/testsuite/
	* gcc.dg/Wstringop-overflow-89.c: New test.