Make sect_offset and cu_offset strong typedefs instead of structs

Make sect_offset and cu_offset strong typedefs instead of structs

A while ago, back when GDB was a C program, the sect_offset and
cu_offset types were made structs in order to prevent incorrect mixing
of those offsets.  Now that we require C++11, we can make them
integers again, while keeping the safety, by exploiting "enum class".
We can add a bit more safety, even, by defining operators that the
types _should_ support, helping making the suspicious uses stand out
more.

Getting at the underlying type is done with the new to_underlying
function added by the previous patch, which also helps better spot
where do we need to step out of the safety net.  Mostly, that's around
parsing the DWARF, and when we print the offset for complaint/debug
purposes.  But there are other occasional uses.

Since we have to define the sect_offset/cu_offset types in a header
anyway, I went ahead and generalized/library-fied the idea of "offset"
types, making it trivial to add more such types if we find a use.  See
common/offset-type.h and the DEFINE_OFFSET_TYPE macro.

I needed a couple generaly-useful preprocessor bits (e.g., yet another
CONCAT implementation), so I started a new common/preprocessor.h file.

I included units tests covering the "offset" types API.  These are
mostly compile-time tests, using SFINAE to check that expressions that
shouldn't compile (e.g., comparing unrelated offset types) really are
invalid and would fail to compile.  This same idea appeared in my
pending enum-flags revamp from a few months ago (though this version
is a bit further modernized compared to what I had posted), and I plan
on reusing the "check valid expression" bits added here in that
series, so I went ahead and defined the CHECK_VALID_EXPR macro in its
own header -- common/valid-expr.h.  I think that's nicer regardless.

I was borderline between calling the new types "offset" types, or
"index" types, BTW.  I stuck with "offset" simply because that's what
we're already calling them, mostly.

gdb/ChangeLog:
2017-04-04  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/offset-type-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add offset-type-selftests.o.
	* common/offset-type.h: New file.
	* common/preprocessor.h: New file.
	* common/traits.h: New file.
	* common/valid-expr.h: New file.
	* dwarf2expr.c: Include "common/underlying.h".  Adjust to use
	sect_offset and cu_offset strong typedefs throughout.
	* dwarf2expr.h: Adjust to use sect_offset and cu_offset strong
	typedefs throughout.
	* dwarf2loc.c: Include "common/underlying.h".  Adjust to use
	sect_offset and cu_offset strong typedefs throughout.
	* dwarf2read.c: Adjust to use sect_offset and cu_offset strong
	typedefs throughout.
	* gdbtypes.h: Include "common/offset-type.h".
	(cu_offset): Now an offset type (strong typedef) instead of a
	struct.
	(sect_offset): Likewise.
	(union call_site_parameter_u): Rename "param_offset" field to
	"param_cu_off".
	* unittests/offset-type-selftests.c: New file.