c-family: fix attribute constructor ICE [PR90658]
Here the C compiler crashes because a FUNCTION_DECL got into
get_priority -> default_conversion, and the C FE's version of d_c
specifically asserts that it doesn't get a FUNCTION_DECL. All uses
of default_conversion in c-attribs.cc are guarded by != IDENTIFIER_NODE
&& != FUNCTION_DECL, but get_priority was only checking IDENTIFIER_NODE.
PR c/90658
gcc/c-family/ChangeLog:
* c-attribs.cc (get_priority): Check FUNCTION_DECL.
gcc/testsuite/ChangeLog:
* c-c++-common/attr-cdtor-1.c: New test.