[Leitwolf11]

Link LLD's COFF, ELF and Mach-O linkers into talac

talac started lld-link, cc or clang as separate programs to link an
executable, and wrote the import libraries of a Windows executable by
running lld-link /lib. It now links lldCOFF, lldELF, lldMachO and
lldCommon statically and runs them in its own process.

• A Windows executable is linked by LLD's lld-link with the plan's
  arguments and needs nothing from the host; its import libraries
  (kernel32 for TalaRT's archive, talart.dll for the shared library) are
  written by llvm::object::writeImportLibrary.
• A Linux, macOS or BSD executable needs the start-up objects and the C
  library of its target, which a C compiler knows the places of. talac
  runs clang -### with the plan's arguments, which prints the command
  line of the linker without running anything, and runs that command line
  with LLD's ELF or Mach-O linker. A cross target asks clang with
  --target; a host target is given its triple too. When no clang is on
  PATH, a host target is linked by cc and its own linker as before.
• LinkerCommandOf reads the linker's command line from the driver's
  output (quoted words, backslash escapes, the last command whose program
  is a linker); LinkPlanTests read clang output of Linux, macOS and a
  Windows host.

Building: llvm.bzl finds LLD's libraries and headers beside LLVM or in
LLD_ROOT (brew --prefix lld on macOS), adds the LLVM components LLD
uses (all targets, LTO, DTLTO where LLVM has it, debug info, option,
TextAPI, the Windows drivers and manifest), and exposes @llvm//:lld with
the libraries in link order. LLVM's release for Windows names zlib, zstd
and libxml2 among its system libraries without shipping them, so the
Windows build takes them from the Bazel Central Registry. The macOS job
installs the lld formula. The Linux hosts need the libc++ toolchain with
LLD (#115) pinned before this can build there.

LldTests, a new native suite, write import libraries for x64 and arm64,
check that a refused link reports LLD's messages and that LLD links again
afterwards, and link a Windows executable from an object LLVM generates in
the test; on a Windows host the executable runs and returns its entry's
42. Locally on Windows: lld_tests 3/12, link_plan_tests 9/103,
verify-abi and the Go helper tests pass.

[Leitwolf11]

Self-review: LLD's COFF, ELF and Mach-O linkers are linked into talac statically, and Lld.cpp is the only place that calls them, through lld::lldMain. Import libraries are written by LLVM's writeImportLibrary. A Windows target needs nothing from the host. ELF and Mach-O take the linker line from clang -###, read by LinkerCommandOf, which is pure and independent of the host's path rules, and run it in-process; without clang a target of the host falls back to cc. The build takes LLD's static libraries from the LLVM release on Windows, from toolchain revision 2 on Linux, and from a cached static build against Homebrew's LLVM on macOS, because Homebrew's lld is shared over libLLVM. Linux digests are pinned in the helpers and both Containerfiles. Known limits: the cross path still passes -fuse-ld=lld to clang -###, which may fail where no ld.lld is installed, and runtimes/<triple>/ is not staged. CI 40/40 green.