Don't mangle attributes that have a space in their name

Don't mangle attributes that have a space in their name

The SVE port needs to maintain a different type identity for
GNU vectors and "SVE vectors" even during LTO, since the types
use different ABIs.  The easiest way of doing that seemed to be
to use type attributes.  However, these type attributes shouldn't
be user-facing; they're just a convenient way of representing the
types internally in GCC.

There are already several internal-only attributes, such as "fn spec"
and "omp declare simd".  They're distinguished from normal user-facing
attributes by having a space in their name, which means that it isn't
possible to write them directly in C or C++.

Taking the same approach mostly works well for SVE.  The only snag
I've hit so far is that the new attribute needs to (and only exists to)
affect type identity.  This means that it would normally get included
in mangled names, to distinguish it from types without the attribute.

However, the SVE ABI specifies a separate mangling for SVE vector types,
rather than using an attribute mangling + a normal vector mangling.
So we need some way of suppressing the attribute mangling for this case.

There are currently no other target-independent or target-specific
internal-only attributes that affect type identity, so this patch goes
for the simplest fix of skipping mangling for attributes whose names
contain a space (which usually wouldn't give a valid symbol anyway).
Other options I thought about were:

(1) Also make sure that targetm.mangled_type returns nonnull.

(2) Check directly for the target-specific name.

(3) Add a new target hook.

(4) Add new information to attribute_spec.  This would be very invasive
    at this stage, but maybe we should consider replacing all the boolean
    fields with flags?  That should make the tables slightly easier to
    read and would make adding new flags much simpler in future.

2020-01-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/cp/
	* mangle.c (mangle_type_attribute_p): New function, split out from...
	(write_CV_qualifiers_for_type): ...here.  Don't mangle attributes
	that contain a space.

From-SVN: r279952