Begin building libc++ and libc++abi runtimes on demand

[chandlerc]

This builds on the previous work to flesh out more on-demand runtimes
building. It adds building of the libc++.a archive runtime.

A number of changes are required for this to work:

• The runtimes build infrastructure needs to support building sources
  from multiple parts of LLVM rather than a single part. We do this by
  lifting the root of the runtimes source paths up a level to a common
  runtimes tree, and installing the runtimes sources below this
  directory.

• Both libc++ and libc++abi runtimes sources need to be installed, and
  we even need to install some interesting parts of llvm-libc that are
  used in the build of libc++.

• We need to generate the site configuration header file for libc++ from
  the CMake template. This includes both setting up a set of
  platform-independent defines and introducing some basic Bazel support
  for processing the CMake template itself.

Doing all of this also exposed some missing features and limitations of
the runtimes building infrastructure that are addressed here.

One note is that all of this just adds libc++ to the explicit
build-runtimes command for testing. It doesn't yet trigger
automatically building these prior to linking, or configuring any of the
other subcommands to automatically use these runtimes. All of that will
come in follow-up PRs.

Also, this makes the clang_runtimes_test ... very slow in our
default build configuration. Compiling libc++, even with many threads on
a large Linux server requires up to 50 seconds. I'm open to any
suggestions on how to handle this, including disabling the test in
non-optimized builds. I have some ideas to speed this up, but
fundamentally building libc++ is... not cheap.

I did look at some of the existing Bazel tools to process the CMake
template, but they all seemed significantly more complex than what we
need and didn't have broad adoption. Given that, it seemed slightly
better to just roll our own given the simple format.

Two of the new LLVM patch are currently under review upstream and so
hopefully temporary:

• Add a filegroup containing _all_ sources to the libc build rules llvm/llvm-project#169155
• [bazel][libc] Remove target compatibility restrictions for float128 llvm/llvm-project#169292

[zygoloid]

This seems like a fairly large chunk of C++ code to be including in a build rule. Would it make sense to move the contents of _LIBCPP_ABI_DEFINES out into a separate header file and add a #include here instead?

(Admittedly that only helps a little, as the remaining cmake defines are equally large.)

[chandlerc]

I'm a bit torn about this...

On one hand, separating out seems much cleaner as the C++ code is no longer embedded in a BUILD file.

On the other hand, there are complex interactions between this C++ code and the defines here, and moving those further apart seems unfortunate.

But seems worth exploring in a follow-up. I've left a TODO to document that this should be revisited and I'll prep a PR we can look at concretely.

[chandlerc]

Thanks for review, merging!

Will send follow-ups to tidy up TODOs and the next steps.

[chandlerc]

I'm a bit torn about this...

On one hand, separating out seems much cleaner as the C++ code is no longer embedded in a BUILD file.

On the other hand, there are complex interactions between this C++ code and the defines here, and moving those further apart seems unfortunate.

But seems worth exploring in a follow-up. I've left a TODO to document that this should be revisited and I'll prep a PR we can look at concretely.