build-script: Enable module builds by default

[vedantk]

We added build-script support for building llvm with modules a while ago. We deferred turning this on by default because we were concerned it would break the Linux bots, or builds of swift-compiler-rt. Try again, now that we've done another stable merge.

[vedantk]

@swift-ci Please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 30c2169
Test requested by - @vedantk

[vedantk]

The failing tests on Linux are:

Failing Tests (2):
Swift(linux-x86_64) :: Reflection/typeref_lowering.swift
Swift(linux-x86_64) :: stdlib/Dictionary.swift

Based on the output, I'm really not sure that this has anything to do with --llvm-enable-modules...

Edit: These tests are failing on the ubuntu/master bot as well.

[vedantk]

@swift-ci Please test Linux platform

[vedantk]

@swift-ci Please smoke test Linux platform

[vedantk]

@adrian-prantl All checks pass, here we go!

[adrian-prantl]

Awesome!

[jrose-apple]

Turns out this broke the ASan build, because sanitizers being on or off isn't properly encoded in the module hash. Can we turn this off automatically for now when ASan is enabled?

[vedantk]

Yikes, yes I'll work up a patch.

[vedantk]

Done in PR 4861.