[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/SwiftLangSupport.cpp

@@ -297,7 +297,8 @@ SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
                                  SKCtx.getGlobalConfiguration());
 
   CompileManager = std::make_shared<compile::SessionManager>(
-      RuntimeResourcePath, DiagnosticDocumentationPath, Plugins);
+      SwiftExecutablePath, RuntimeResourcePath, DiagnosticDocumentationPath,
+      Plugins);
 
   // By default, just use the in-memory cache.
   CCCache->inMemory = std::make_unique<ide::CodeCompletionCache>();