Fix BLOCK_COMMENT in the context of SHEBANG

This fixes an issue with the BLOCK_COMMENT grammar as it is used within
the SHEBANG rule. The problem is that BLOCK_COMMENT was changed in
https://github.com/rust-lang/reference/pull/2191 so that it relied on
the order of how the comments were handled in the COMMENT rule to avoid
ambiguity with OUTER_BLOCK_DOC. However, because SHEBANG is using the
BLOCK_COMMENT rule directly, we need BLOCK_COMMENT to be able to stand
on its own without relying on the order in COMMENT.

This fixes an issue with shebang such as:

	#! /** doc */ [attr]

Here this should be treated as a shebang (because it is a doc block
comment).

The solution is to do the appropriate negative lookahead to exclude
OUTER_BLOCK_DOC and INNER_BLOCK_DOC.