[build-script] Add a flag for sccache

[porglezomp]

This adds a flag to enable sccache in order to simplify the build-script invocation, particularly for new contributors.

The implementation is based on the implementation of the --distcc flag.
CC @gottesmm

[shahmishal]

Thanks for doing this!

[shahmishal]

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[gottesmm]

Thanks!

[shahmishal]

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[shahmishal]

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[varungandhi-apple]

Maybe I'm missing something in the PR, but what part of the code is finding the path to sccache and setting toolchain.sccache to that path?

[porglezomp]

The toolchains do a generic lazy lookup for tools, so this just adds this line that makes it eligible:

_register("sccache", "sccache")

[shahmishal]

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[varungandhi-apple]

Potential addition: Should this be checking if sccache's server is not on, and turn it on if that's not the case?

[porglezomp]

According to the sccache README, that shouldn't be necessary:

The sccache command will spawn a server process if one is not already running

[varungandhi-apple]

Nice, I didn't realize that. We could get rid of the --start-server in the Getting Started guide then.

[edymtt]

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[shahmishal]

00:03:20.173 ./utils/swift_build_support/tests/test_cmake.py:135:13: E128 continuation line under-indented for visual indent

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