c: ICE on invalid with attribute optimize [PR115549]

c: ICE on invalid with attribute optimize [PR115549]

I had this PR in my open tabs so why not go ahead and fix it.

decl_attributes gets last_decl, the last already pushed declaration,
to be used in common_handle_aligned_attribute.  In C++, we look up
the decl via find_last_decl, which returns NULL_TREE if it finds
a decl that had not been declared.  In C, we look up the decl via
lookup_last_decl which returns error_mark_node rather than NULL_TREE
in that case.

The error_mark_node causes a crash in common_handle_aligned_attribute.
We can fix this on the C FE side like in the patch below.

	PR c/115549

gcc/c/ChangeLog:

	* c-decl.cc (c_decl_attributes): If lookup_last_decl returns
	error_mark_node, use NULL_TREE as last_decl.

gcc/testsuite/ChangeLog:

	* c-c++-common/attr-aligned-2.c: New test.