Detect conflicts between incompatible uses of the same attribute (PR c/78666).

Detect conflicts between incompatible uses of the same attribute (PR c/78666).

Resolves:
PR c/78666 - conflicting attribute alloc_size accepted
PR c/96126 - conflicting attribute section accepted on redeclaration

gcc/c-family/ChangeLog:

	PR c/78666
	PR c/96126
	* c-attribs.c (validate_attr_args): New function.
	(validate_attr_arg): Same.
	(handle_section_attribute): Call it.  Introduce a local variable.
	(handle_alloc_size_attribute):  Same.
	(handle_alloc_align_attribute): Same.

gcc/testsuite/ChangeLog:

	PR c/78666
	PR c/96126
	* gcc.dg/attr-alloc_align-5.c: New test.
	* gcc.dg/attr-alloc_size-13.c: New test.
	* gcc.dg/attr-section.c: New test.
	* c-c++-common/builtin-has-attribute-3.c: Add xfails due to expected
	warnings to be cleaned up.