llvm: various improvements

[carlocab]

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---

1. Enable bootstrap build on Linux. This fails with gcc-5. Let's try again when we switch to Ubuntu 18.04.
2. Clean up some duplication
3. Install the lit command line tool
4. Install Vim editor integration
5. Add patch to fix parallel builds on ARM

The lit tool adds very little to the bottle size, and it seems some
users might find it useful. Let's install it directly using Python to
avoid having to set LLVM_INCLUDE_TESTS to ON, as that has many other
side-effects that we don't need or want. This is also how Arch installs
lit. [1]

Closes #82479.

[1] https://github.com/archlinux/svntogit-packages/blob/5e7a8f09f53d0fb19c0680bb5b4691b1c6a78ad1/trunk/PKGBUILD#L79-L82

[carlocab]

On Linux:

CMake Error at /tmp/llvm-20210809-5953-1takjgm/llvm-project-12.0.1.src/compiler-rt/lib/crt/CMakeLists.txt:71 (message):
  gcc-5: error: unrecognized command line option ‘-Wimplicit-fallthrough’

  gcc-5: error: unrecognized command line option
  ‘-Wcovered-switch-default’

  gcc-5: error: unrecognized command line option ‘-Wstring-conversion’

Call Stack (most recent call first):
  /tmp/llvm-20210809-5953-1takjgm/llvm-project-12.0.1.src/compiler-rt/lib/crt/CMakeLists.txt:92 (check_cxx_section_exists)

Enabling the bootstrap on Linux might have to wait until we switch to Ubuntu 18.04.

[SMillerDev]

This will always be ON on CI I think

[carlocab]

Do you mean LLVM_CREATE_XCODE_TOOLCHAIN? That's true. I'll move it out of the on_macos block so we don't need to declare it in two places.

[danielnachun]

On Linux:

CMake Error at /tmp/llvm-20210809-5953-1takjgm/llvm-project-12.0.1.src/compiler-rt/lib/crt/CMakeLists.txt:71 (message):
  gcc-5: error: unrecognized command line option ‘-Wimplicit-fallthrough’

  gcc-5: error: unrecognized command line option
  ‘-Wcovered-switch-default’

  gcc-5: error: unrecognized command line option ‘-Wstring-conversion’

Call Stack (most recent call first):
  /tmp/llvm-20210809-5953-1takjgm/llvm-project-12.0.1.src/compiler-rt/lib/crt/CMakeLists.txt:92 (check_cxx_section_exists)

Enabling the bootstrap on Linux might have to wait until we switch to Ubuntu 18.04.

I don't want to waste too much time hunting this down right now, but I'm puzzled as to why this fails when we are actually building compiler-rt with gcc-5 at some point. If there was a way to drop these flags from being added, then it would probably work. But we don't have to hold up progress on the rest of this just to get this figured out.

[carlocab]

That's not true -- we've never built compiler-rt with gcc-5. Recall that we pass compiler-rt in LLVM_ENABLE_RUNTIMES and not LLVM_ENABLE_PROJECTS.

[carlocab]

Linux runner timed out at testing dependents. This probably needs to be run on the self-hosted runner now.

[carlocab]

I think it'll be good to wait until Homebrew/brew#10769 is merged for this, as it'll be useful to rebuild llvm when we change the Big Sur SDK path.

Also, I should probably write a proper test for lit, as I think this formula being keg-only could make things a bit weird.

[carlocab]

Incorporated into #84364.