[FrontendOptions] Default IndexStoreCompress to false

Author: ahoppen

include/swift/Frontend/FrontendOptions.h

@@ -137,7 +137,7 @@ class FrontendOptions {
   bool IndexIncludeLocals = false;
 
   /// Whether to compress the record and unit files in the index store.
-  bool IndexStoreCompress;
+  bool IndexStoreCompress = false;
 
   bool SerializeDebugInfoSIL = false;
   /// If building a module from interface, ignore compiler flags