[SourceKit] Pass 'swiftc' path to Driver when creating frontend args

Driver uses its path to derive the plugin paths (i.e.
'lib/swift/host/plugins' et al.) Previously it was a constant string
'swiftc' that caused SourceKit failed to find dylib plugins in the
toolchain. Since 'SwiftLangSupport' knows the swift-frontend path,
use it, but replacing the filename with 'swiftc', to derive the plugin
paths.

rdar://107849796

tools/SourceKit/lib/SwiftLang/SwiftCompile.cpp

@@ -31,9 +31,10 @@ compile::SessionManager::getSession(StringRef name) {
   }
 
   bool inserted = false;
-  std::tie(i, inserted) = sessions.try_emplace(
-      name, std::make_shared<compile::Session>(
-                RuntimeResourcePath, DiagnosticDocumentationPath, Plugins));
+  std::tie(i, inserted) =
+      sessions.try_emplace(name, std::make_shared<compile::Session>(
+                                     SwiftExecutablePath, RuntimeResourcePath,
+                                     DiagnosticDocumentationPath, Plugins));
   assert(inserted);
   return i->second;
 }