Pass through shell script build phase environment and working directory for build tool commands

[abertelrud]

Use the new LLBuildManifestWriter support to pass through shell script build phase environment and working directory for build tool commands.

This PR builds on #3332

Motivation:

Plugin-created commands should be able to specify environment and initial working directory for those commands.

Modifications:

• make build tool commands write out the llbuild environment entries and initial working directory path using the support added in Extend LLBuildManifestWriter to let shell commands specify environment and initial working directory #3332
• add a unit test that requires both of these features in order to build successfully

rdar://74663489

[abertelrud]

@swift-ci please smoke test

[abertelrud]

Note: This should only be merged after #3332 since every PR is a squash now but the changes should be separate in the revision history.

[abertelrud]

error: command Generating foo.swift from foo.dat failed: working-directory unsupported on this platform

Ok, that's disappointing. That will require an llbuild fix.

[abertelrud]

Oh right, it's because it uses pthread_chdir_np() which is only supported on Darwin. In this case we can probably work around it by wrapping it in a shell and prepending a cd.

[abertelrud]

It's a little annoying — supporting initial working directory is mostly for completeness. Build tools should in general take full paths and not rely on a custom working directory. We'll need to decide whether supporting relative paths (arguably an anti-pattern) is important enough to put in workarounds, or whether it's better to leave it out and just have the plugin form the path.

Environment is the important one to support.

[abertelrud]

I think what I'm going to do here is to remove the working directory functionality for now. It's not so important as to steal time from other things (such as better diagnostics, etc), and it is straightforward to add later.

[artemcm]

@swift-ci please smoke test linux

[abertelrud]

Updated to mark the ability to set a working directory as not yet available, and updated unit test.

[abertelrud]

@swift-ci please smoke test