`-disable-sandbox` to disable sandboxing when invoking subprocess from
from the frontend. Since `sandbox(7)` in macOS doesn't support nested
sandbox, complation used to fail when the parent build process is sandboxed.
Instead of emitting an warning to the diagnostic engine, return the
plugin loading error as the result of the request. So that the user
can decide to emit it as a warning or an error.
Iterating all options and potential file system access is not great for
every plugin lookup request. Instead, lazily create a single lookup table
keyed by module name.
'load-plugin-library', 'load-plugin-executable', '-plugin-path' and
'-external-plugin-path' should be searched in the order they are
specified in the arguments.
Previously, for example '-plugin-path' used to precede
'-external-plugin-path' regardless of the position in the arguments.
* Factor out ASTContext plugin loading to newly introduced 'PluginLoader'
* Insert 'DependencyTracker' to 'PluginLoader'
* Add dependencies right before loading the plugins
rdar://104938481
