Two minor, related fixes for template instantiation with blocks:

  - Make sure that the block expression is instantiation-dependent if the
    block is in a dependent context
  - Make sure that the C++ 'this' expression gets captured even if we
  don't rebuild the AST node during template instantiation. This would
  also have manifested as a bug for lambdas.

Fixes <rdar://problem/10832617>.

llvm-svn: 151372