[BitcodeAnalyzer] allow a motivated user to dump BLOCKINFO

This adds the `--dump-blockinfo` flag to `llvm-bcanalyzer`, allowing a sufficiently motivated user to dump (parts of) the `BLOCKINFO_BLOCK` block. The default behavior is unchanged, and `--dump-blockinfo` only takes effect in the same context as other flags that control dump behavior (i.e., requires that `--dump` is also passed).

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D107536