[CodeGen][ObjC] Build the global block structure before emitting the

body of global block invoke functions.

This commit fixes an infinite loop in IRGen that occurs when compiling
the following code:

void FUNC2() {
  static void (^const block1)(int) = ^(int a){
    if (a--)
      block1(a);
  };
}

This is how IRGen gets stuck in the infinite loop:

1. GenerateBlockFunction is called to emit the body of "block1".

2. GetAddrOfGlobalBlock is called to get the address of "block1". The
   function calls getAddrOfGlobalBlockIfEmitted to check whether the
   global block has been emitted. If it hasn't been emitted, it then
   tries to emit the body of the block function by calling
   GenerateBlockFunction, which goes back to step 1.

This commit prevents the inifinite loop by building the global block in
GenerateBlockFunction before emitting the body of the block function.

rdar://problem/34541684

Differential Revision: https://reviews.llvm.org/D38118

llvm-svn: 314029