This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.
Updating the vendor area to match this layout is next.
* Bootstrap llvm-tblgen and clang-tblgen with a minimal llvm static
library, that has no other dependencies.
* Roll up all separate llvm libraries into one big static libllvm.
* Similar for all separate clang and lldb static libraries.
* For all these libraries, generate their .inc files only once.
* Link all llvm tools (including extra) against the big libllvm.
* Link clang and clang-format against the big libllvm and libclang.
* Link lldb against the big libllvm, libclang and liblldb.
N.B.: This is work in progress, some details may still be missing.
It also heavily depends on bsd.*.mk's support for SRCS and DPSRCS with
relative pathnames, which apparently does not always work as expected.
For building llvm, clang and lldb though, it seems to work just fine.
The main idea behind this restructuring is maintainability and build
peformance. The previous large number of very small libraries, each
with their own generated files and dependencies was slow to traverse
and hard to understand.
Possible future improvements:
* Only build certain targets, e.g. for most regular users having just
one target will be fine. This will shave off some build time.
* Building the big llvm, clang and lldb libraries as shared (private)
libraries.
* Adding other components from the LLVM project, such as lld.
