Let hhbbc know that invariant_violation always fails

Summary: hhbbc does not know about call_user_func_array(), so when

  invariant(condition(), ...);

is rewritten as

  if (!condition()) {
    invariant(false, ...);
  }

it has trouble figuring out that invariant(false, ...) never
falls through.

After this diff, hhbbc figures out correctly that invariant(false, ...)
returns TBottom, but still cannot fully use this knowledge.

Reviewed By: @ptarjan, @elgenie

Differential Revision: D1595071
Kill HHIREnableCalleeSaveOpt runtime option

Summary: It isn't used anymore.

Reviewed By: @swtaarrs

Differential Revision: D1593488
Use precomputed static strings for chr

Summary: We already have these to support $str[$idx]. Might as
well use them for chr($chr) too.

Reviewed By: @swtaarrs

Differential Revision: D1595026
Allow shutdown function registration during shutdown

Summary: Closes #2981

Reviewed By: @ptarjan

Differential Revision: D1561536
Fix typo in builtins_async_mysql.idl.hhi

Summary: This was blocking the Codex parser from indexing the file.

Reviewed By: @jwatzman

Differential Revision: D1588349