Parse HHBC option as bool only if it's a flag

Parse HHBC option as bool only if it's a flag

Summary:
The JSON config passed to hh_single_compile by HHVM contains a
**superset** of HHBC-relevant config, e.g.:

  { ...
    "hhvm.trusted_db_path": {
       "access": 4,
       "local_value": "",
       "global_value": "" }
  }

Therefore, skip the interpretation of such an option as flag in Rust
**until *after*** it's determined that it indeed represents a flag.
Now, unrelated HHVM options with values such as:

- "" (empty string); or
- "12345678901234567890" (integer-overflowing constant)

no longer panic / make the whole JSON parse to fail
(as a result of parsing a non-int/bool-convertible string).

Add regression tests for these two cases.

Finally, treat empty string values as false for flags as in OCaml code.

Reviewed By: dabek

Differential Revision: D20265758

fbshipit-source-id: 74cac1d8a718c35a06a804aee5f9a97268f3ef30