[CodeGen][ObjC] Convert '[self alloc]' in a class method to a call to

'objc_alloc(self)'

Also convert '[[self alloc] init]' in a class method to a call to
'objc_alloc_init(self)'.

rdar://problem/50855121

Differential Revision: https://reviews.llvm.org/D62643

llvm-svn: 362521