This involved adding a new substitution called %api_diff_data_dir that when
building against a host toolchain, looks in the host toolchain (next to swiftc)
rather than in the resource dir. The reason why I need to do this is this allows
me to perform a stdlib stage2 build without needing to build swift itself.
The only interesting changes here are that I had to add %api_diff_data_dir to
a bunch of normal/expected tests and also add %api_diff_data_dir's length to the
offsets in rdar31892850.swift.
Introduce `repairByExplicitRawRepresentativeUse` which is used for
assignment, argument-to-parameter conversions and contextual mismatches.
It checks whether `to` side is a raw representable type and tries
to match `from` side to its `rawValue`.
When we determine that an optional value needs to be unwrapped to make
an expression type check, use notes to provide several different
Fix-It options (with descriptions) rather than always pushing users
toward '!'. Specifically, the errors + Fix-Its now looks like this:
error: value of optional type 'X?' must be unwrapped to a value of
type 'X'
f(x)
^
note: coalesce using '??' to provide a default when the optional
value contains 'nil'
f(x)
^
?? <#default value#>
note: force-unwrap using '!' to abort execution if the optional
value contains 'nil'
f(x)
^
!
Fixes rdar://problem/42081852.
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.
