Variable auto-init: fix __block initialization

Summary:
Automatic initialization [1] of __block variables was trampling over the block's
headers after they'd been initialized, which caused self-init usage to crash,
such as here:

  typedef struct XYZ { void (^block)(); } *xyz_t;
  __attribute__((noinline))
  xyz_t create(void (^block)()) {
    xyz_t myself = malloc(sizeof(struct XYZ));
    myself->block = block;
    return myself;
  }
  int main() {
    __block xyz_t captured = create(^(){ (void)captured; });
  }

This type of code shouldn't be broken by variable auto-init, even if it's
sketchy.

[1] With -ftrivial-auto-var-init=pattern

<rdar://problem/47798396>

Reviewers: rjmccall, pcc, kcc

Subscribers: jkorous, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57797

llvm-svn: 353495