Some code at the interface between irregexp and the rest of V8 uses
V8-specific APIs that are not used elsewhere in irregexp. For example,
only a handful of functions in irregexp call or are called from
generated code. When embedding irregexp into SpiderMonkey, these
functions are an obstacle, because they are dead code, but still have
to compile.

To simplify the process of embedding, this patch does two things:

1. It moves StringCharacterPosition out of irregexp and into
objects/string, renaming it "AddressOfCharacterAt".

2. It guards the following set of functions with
'#ifndef COMPILING_IRREGEXP_FOR_EXTERNAL_EMBEDDER':
- IrregexpInterpreter::MatchForCallFromJs
- NativeRegExpMacroAssembler::CheckStackGuardState
- NativeRegExpMacroAssembler::Match
- NativeRegExpMacroAssembler::Execute

This will have no effect in a V8 build, but can be defined by
SpiderMonkey or another embedder to omit the problematic functions.

In the future, if we attempt to make a cleaner separation between V8
and irregexp, these functions will be a good place to start defining
the API boundary.

R=jgruber@chromium.org

Bug: v8:10303
Change-Id: I9f531a36e4f13440cafb0d0ade921f4c09f39c05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2097220Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66750}