This patch adds the concept of "sizeless" types to C and C++.
The documentation part of the patch describes the extension in
detail and explains the rationale. The patch is a prerequisite
for adding AArch64 SVE intrinsic functions.
The main change is to make several checks use the new distinction
between "definite" and "indefinite" types instead of the usual
distinction between "complete" and "incomplete" types. There are
also a couple of places that check specifically for sizeless types.
The patch doesn't reword the diagnostics to talk about indefinite
types rather than incomplete types since (a) "indefinite" won't
mean much to most users and (b) the patch shouldn't affect the user
experience for non-AArch64 targets.
The FIXMEs about inline asms are resolved by later SVE patches.
The change to DiagnoseForRangeVariableCopies is for consistency.
As the tests show, DiagnoseForRangeReferenceVariableCopies can apply
to sizeless types, but at the moment DiagnoseForRangeConstVariableCopies
(which is the only part affected by the completeness check) doesn't
warn for PODs, and so doesn't warn for sizeless types either.