[Frontend] Add a predefined macro that describes the Objective-C bool type

This commit adds a new predefined macro named __OBJC_BOOL_IS_BOOL that describes
the Objective-C boolean type: its value is zero if the Objective-C boolean uses
the signed character type, otherwise its value is one as the Objective-C boolean
uses the builtin boolean type.

rdar://21170440

Differential Revision: https://reviews.llvm.org/D26234

llvm-svn: 287529