Modify IRGen to emit builtin access markers with an error flag in
Swift 3 mode.
KeyPath enforcement is required by user code in Swift 4+ mode, but is
implemented within the standard library. A [builtin] flag marks the
special case for access generated by Builtins so that they are
always enforced as an error regardless of the language mode.
This is necessary for Swift 4.2 because the standard library continues
to build in Swift 3 mode. Once the standard library build migrates,
this is all irrelevant.
This does not actually affect existing Swift 3 code, since the KeyPath
feature wasn't introduced until Swift 4.
<rdar://problem/40115738> [Exclusivity] Enforce Keypath access as an error, not a warning in 4.2.
Add dynamic enforcement of exclusive access when a KeyPath directly accesses a final
stored property on an instance of a class. For read-only projections, this begins and ends
the access immediately. For mutable projections, this uses the ClassHolder to perform
a long-term access that lasts as long as the lifetime of the ClassHolder.
rdar://problem/31972680
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
