Summary:
The frontend of ObjC regarding to captured variable was incorrect: it set capturing mode as
by-reference always, but it actually translaged as if all captured variables were passed with
by-value. This diff fixes this based on the document.
https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Blocks/Articles/bxVariables.html
* global variable: by reference
* local variable: by value
* `static` local variable: by reference
* `__block` local variable: by reference
* parameter: by value
Reviewed By: jvillard
Differential Revision: D25306122
fbshipit-source-id: ec499d705
Sungkeun Cho