[ObjC Availability] Add missing const to getVersion function of ObjCAvailabilityCheckExpr class

[lcs]

Summary

Add missing const to getVersion function of ObjCAvailabilityCheckExpr class.

This feels like a bug on the original change D22171. We cannot really call this function from a const object pointer because the function is not marked as const.

This diff adds the missing const specifier to fix the issue.

clang/include/clang/AST/ExprObjC.h

explicit ObjCAvailabilityCheckExpr(EmptyShell Shell)
       : Expr(ObjCAvailabilityCheckExprClass, Shell) {}
 
   SourceLocation getBeginLoc() const { return AtLoc; }
   SourceLocation getEndLoc() const { return RParen; }
   SourceRange getSourceRange() const { return {AtLoc, RParen}; }
 
   /// This may be '*', in which case this should fold to true.
   bool hasVersion() const { return !VersionToCheck.empty(); }
-  VersionTuple getVersion() { return VersionToCheck; }
+  VersionTuple getVersion() const { return VersionToCheck; }
 
   child_range children() {
     return child_range(child_iterator(), child_iterator());
   }
 
   const_child_range children() const {
     return const_child_range(const_child_iterator(), const_child_iterator());
   }