precompile and cache Carbon prelude (#7432)

Refactors the link driver to automatically compile and cache the carbon
prelude for use in linking.

Implements a `carbon_library` rule for compiling the Core library
dependencies in the examples.

toolchain/diagnostics/coverage_test.cpp

@@ -36,10 +36,12 @@ constexpr Kind UntestedKinds[] = {
     Kind::BuildPreludeManifestError,
     Kind::BuildTempDirectoryCreationError,
     Kind::BuildTempDirectoryDeletionError,
+    Kind::CompileCoreManifestError,
     Kind::CompilePreludeManifestError,
     Kind::ConfigFailedToReadDigest,
     Kind::ConfigFailedToSetupTarget,
     Kind::DriverInstallInvalid,
+    Kind::LinkCarbonPreludeBuildFailed,
 
     // These diagnose filesystem issues that are hard to unit test.
     Kind::ErrorReadingFile,