add first implementation of carbon build subcommand (#7239)

This is the first draft of the implementation of
[p6333](https://docs.carbon-lang.dev/proposals/p6333.html).

The `build` subcommand shared logic with the `compile` and `link`
subcommands,
so I've moved some of the functionality in `compile` and `link` to
shared
`CompileDriver` and `LinkDriver` classes, respectively. This also
required exposing the
`CompileOptions` and `LinkOptions` subcommand structs for re-use.

There's still some work to do on the proposal, most notably the package
include automatic path resolution and import, and the refactors to
`carbon compile`.

toolchain/diagnostics/coverage_test.cpp

@@ -31,6 +31,11 @@ constexpr Kind UntestedKinds[] = {
 
     // Diagnosing erroneous install conditions, but test environments are
     // typically correct.
+    Kind::BuildFailureRunningClangToLink,
+    Kind::BuildOutputFileOpenError,
+    Kind::BuildPreludeManifestError,
+    Kind::BuildTempDirectoryCreationError,
+    Kind::BuildTempDirectoryDeletionError,
     Kind::CompilePreludeManifestError,
     Kind::ConfigFailedToReadDigest,
     Kind::ConfigFailedToSetupTarget,