Revert "[Caching][NFC] Using llvm::cas::CASConfiguration"

This reverts commit 4f059033bb. The change
is actually not NFC since previously, there is a cache in the
CompilerInvocation that prevents the same CAS from the same CASOptions
from being initialized multiple times, which was relied upon when
running inside sub invocation. When switching to a non-caching simple
CASOption types, it causes every single sub instance will create its own
CAS, and it can consume too many file descriptors and causing errors
during dependency scanning.

rdar://164903080

lib/Frontend/Frontend.cpp

@@ -477,12 +477,12 @@ bool CompilerInstance::setupCASIfNeeded(ArrayRef<const char *> Args) {
     return false;
 
   const auto &Opts = getInvocation().getCASOptions();
-  if (Opts.Config.CASPath.empty() && Opts.Config.PluginPath.empty()) {
+  if (Opts.CASOpts.CASPath.empty() && Opts.CASOpts.PluginPath.empty()) {
     Diagnostics.diagnose(SourceLoc(), diag::error_cas_initialization,
                          "no CAS options provided");
     return true;
   }
-  auto MaybeDB = Opts.Config.createDatabases();
+  auto MaybeDB = Opts.CASOpts.getOrCreateDatabases();
   if (!MaybeDB) {
     Diagnostics.diagnose(SourceLoc(), diag::error_cas_initialization,
                          toString(MaybeDB.takeError()));