(I'm trying to get a bootstrap self-build on Windows, where lld is used as a
linker for the stage2.)
I assume BOOTSTRAP_LLVM_ENABLE_LLD works the same way as LLVM_ENABLE_LLD,
but enables to use just-built lld for the next stage.
The issue with LLVM_ENABLE_LLD is that it just passes -fuse-ld=lld
to compiler/linker options which makes sense only for those platforms
where cmake invokes a compiler driver for linking.
On Windows cmake invokes a linker directly and requires CMAKE_LINKER
option to be specified otherwise it defaults CMAKE_LINKER to be link.exe.
Passing CMAKE_LINKER is easy for 1-stage builds, but it's notquite handy
to do for multistage builds, because it's not possible to know for sure
where to find just-built lld.
This patch allows BOOTSTRAP_LLVM_ENABLE_LLD to set CMAKE_LINKER in the case
of building for host Windows. It also skips adding '-fuse-ld=lld' to make
lld-link not warning about 'unknown argument'. The latter part is taken
from https://reviews.llvm.org/D69030 with additional checks that
CMAKE_LINKER doesn't contain a path to another compiler.