compiler: improve error reporting

The functions `Compilation.create` and `Compilation.update` previously
returned inferred error sets, which had built up a lot of crap over
time. This meant that certain error conditions -- particularly certain
filesystem errors -- were not being reported properly (at best the CLI
would just print the error name). This was also a problem in
sub-compilations, where at times only the error name -- which might just
be something like `LinkFailed` -- would be visible.

This commit makes the error handling here more disciplined by
introducing concrete error sets to these functions (and a few more as a
consequence). These error sets are small: errors in `update` are almost
all reported via compile errors, and errors in `create` are reported
through a new `Compilation.CreateDiagnostic` type, a tagged union of
possible error cases. This allows for better error reporting.

Sub-compilations also report errors more correctly in several cases,
leading to more informative errors in the case of compiler bugs.

Also fixes some race conditions in library building by replacing calls
to `setMiscFailure` with calls to `lockAndSetMiscFailure`. Compilation
of libraries such as libc happens on the thread pool, so the logic must
synchronize its access to shared `Compilation` state.

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