[ObjC] Enter a new evaluation context before calling

BuildBlockForLambdaConversion.

Previously, clang would build an incorrect AST for the following code:

id test() {
  return @{@"a": [](){}, @"b": [](){}};
}

ReturnStmt 0x10d080448
`-ExprWithCleanups 0x10d080428
  |-cleanup Block 0x10d0801f0 // points to the second BlockDecl
    ...
    -BlockDecl 0x10d07f150 // First block
    ...
    -BlockDecl 0x10d0801f0 // Second block
    ...
     `-ExprWithCleanups 0x10d0801d0
       |-cleanup Block 0x10d07f150 // points to the first BlockDecl

To fix the bug, this commit enters a new evaluation context to reset
ExprNeedsCleanups before each block is parsed.

rdar://problem/16879958

Differential Revision: http://reviews.llvm.org/D18815

llvm-svn: 268527