Embedded Swift must defer some of the diagnostics about its
restrictions to late in the optimization pipeline, because some
source-level constructs (such as metatypes) are allowed in specific
cases that cannot be diagnosed in the type checker.
Put these diagnostics into the same EmbeddedRestrictions diagnostic
group as the type checker diagnostics, because that's the central
point at which we can direct folks to more information about the
restrictions in Embedded Swift.
Addresses rdar://119383905.
c8e6bfd814