Adopt llbuilds new env-bases section to avoid duplicate envs across commands

[rconnell9]

This is change 2/3 of the adoption of llbuilds new in-memory manifest transfer API.

This change adopts llbuilds new env-bases section to avoid passing duplicate environment variables across commands.

This change does not impact the high level build description. The build description continues to maintains the linkage between each task and its environment. Prior to writing the manifest we scan the environments of each task, adding them (if not a duplicate) as a top-level entry to the llbuild manifest. When commands are written, overrides are computed and inserted when required.

This change should have not impact on the command signature that llbuild computes (the env is simply pulled from a different location in llbuild). In other words this change should not require re-build.

Project	Commands	Before	After	Reduction	Bases declared	Commands using one
WebKit (WebKit.xcworkspace)	37,301	429.7 MiB	85.0 MiB	−80.2% (−344.7 MiB)	34	4,064
llbuild (llbuild.xcodeproj)	4,010	9.635 MiB	9.455 MiB	−1.9% (−185 KiB)	3	63

llbuild manifest parse (llbuild buildsystem parse), 10 runs

Project	p50 before	p50 after	Δ p50	p90 before	p90 after	Δ p90
WebKit	2.292s	0.918s	−59.9%	2.298s	0.922s	−59.9%
llbuild	0.052s	0.051s	−1.9%	0.053s	0.052s	−1.9%

[rconnell9]

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[rconnell9]

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[rjmansfield]

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[rjmansfield]

Just some nits.

[rconnell9]

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[rconnell9]

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[rconnell9]

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[rconnell9]

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