[Index] Add an option to compress the record and unit files

Companion of https://github.com/swiftlang/llvm-project/pull/10977.

tools/SourceKit/include/SourceKit/Core/LangSupport.h

@@ -922,6 +922,7 @@ struct IndexStoreOptions {
   bool IgnoreStdlib = false;
   bool DisableImplicitModules = false;
   bool IncludeLocals = false;
+  bool Compress = false;
 };
 
 struct IndexStoreInfo{};