Enable ipa-sra with fnspec attributes

Enable some ipa-sra on fortran by allowing signature changes on functions
with "fn spec" attribute when ipa-modref is enabled.  This is possible since ipa-modref
knows how to preserve things we trace in fnspec and fnspec generated by fortran forntend
are quite simple and can be analysed automatically now.  To be sure I will also add
code that merge fnspec to parameters.

This unfortunately hits bug in ipa-param-manipulation when we remove parameter
that specifies size of variable length parameter. For this reason I added a hack
that prevent signature changes on such functions and will handle it incrementally.

I tried creating C testcase but it is blocked by another problem that we punt ipa-sra
on access attribute.  This is optimization regression we ought to fix so I filled
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103223.

As a followup I will add code classifying the type attributes (we have just few) and
get stats on access attribute.

gcc/ChangeLog:

	* ipa-fnsummary.c (compute_fn_summary): Do not give up on signature
	changes on "fn spec" attribute; give up on varadic types.
	* ipa-param-manipulation.c: Include attribs.h.
	(build_adjusted_function_type): New parameter ARG_MODIFIED; if it is
	true remove "fn spec" attribute.
	(ipa_param_adjustments::build_new_function_type): Update.
	(ipa_param_body_adjustments::modify_formal_parameters): update.
	* ipa-sra.c: Include attribs.h.
	(ipa_sra_preliminary_function_checks): Do not check for TYPE_ATTRIBUTES.

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