When a value is initialized or coerced for a type that conforms to
one of the `ExpressibleBy*Literal` protocols (or
`ExpressibleByStringInterpolation`), this change records an implicit
call to the corresponding `init(...Literal:)` in the indexstore,
located at the beginning of the literal.
This has been an unnecessary code path for a long time now and should be removed particularly because it triggers wasteful `stat` calls.
rdar://51523161
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
