Ninja: Cache target-level flags

Instead of figuring out target flags per-source file, cache the flags
that are being used. This results in a *much* faster generate time for
Ninja.
Revert custom-parsers

Too many corner cases :( .
Merge topic 'revert/custom-parsers' into next

fead0cff Revert custom-parsers
Merge topic 'dev/ninja-speedup' into next

9ba3f771 Ninja: Cache target-level flags
Revert custom-parsers more completely
Merge branch 'revert/custom-parsers' into next