Introduce LLD_BOOTSTRAP to control lld as bootstrap linker

[emaste]

Summary

Add WITH_LLD_BOOTSTRAP and WITHOUT_LLD_BOOTSTRAP knobs similar to the Clang bootstrap ones.

The three knobs are:

WITH_LLD -> Builds /usr/bin/ld.lld
WITH_LLD_BOOTSTRAP -> lld is used as the buildworld linker
WITH_LLD_IS_LD -> The installed /usr/bin/ld is a symlink to ld.lld

This is a work in progress, initially posted for confirmation that these knobs are the desired approach.

[emaste]

See also D10310

[emaste]

Makefile.inc1

1667–1670 ↗

(On Diff #26955)

@dim: this could be tightened up later (e.g. we don't need clang-tblgen to build lld), but I think it's fine at first to build more than is strictly necessary, since building LLD without Clang is probably a rather unlikely combination. Do you agree, or do you think I should try to handle that case with as little built as possible?

[dim]

Makefile.inc1

1667 ↗

(On Diff #26955)

s/llld/lld/ here.

1667–1670 ↗

(On Diff #26955)

Indeed, we don't have separate bootstrap targets for llvm-tblgen and clang-tblgen, but we can do that later. For now this is OK. The additional build time for clang-tblgen is not terribly huge, since it has just 9 .cpp files.

1845 ↗

(On Diff #26955)

It's similar here, really. For lld itself you only need libllvm, but building lld and not clang is a very uncommon use case, which can be optimized for later.