The parser is capable of processing full GN grammar. The generator uses
the visitor and the builder patterns. The visitor handles call expressions
(e.g. target declarations), GN expressions (e.g. is_clang && !is_win),
conditional and assignment statements. Builder is used to generate a
particular output. For example, this CL brings in CMakeBuilder for cmake
generation, but same infrastructure can be reused for generation of
amalgamated sources.

Things that are not yet automatically generated:
1) Automatic generation of config variables (from declare_args).
2) Dependency scanning. Since we only have limited number of targets,
   this is not a big issue at the moment.

Followup CLs will include:
- test targets;
- sample targets.

Bug: v8:10724
Change-Id: I0b31910aca67f0e02ab142b2d35f4338345e7f29
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2306790
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69017}