-Warc-retain-cycles: warn at variable initialization as well as assignment.

Specifically, this should warn:

  __block block_t a = ^{ a(); };

Furthermore, this case which previously warned now does not, since the value
of 'b' is captured before the assignment occurs:

  block_t b; // not __block
  b = ^{ b(); };

(This will of course warn under -Wuninitialized, as before.)

<rdar://problem/11015883>

llvm-svn: 163962