Update documentation and implementation of stage3 build

Have the build work out of the box by forcing an LLD build.

That way, we don't require an external LTO-aware linker,
as we build one.

Also remove reference to the seemingly dead builder.

Differential Revision: https://reviews.llvm.org/D88990

Author: Serge Guelton

clang/cmake/caches/3-stage-base.cmake

@@ -3,6 +3,15 @@ set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
 set(LLVM_BUILD_EXTERNAL_COMPILER_RT ON CACHE BOOL "")
 set(BOOTSTRAP_LLVM_ENABLE_LTO ON CACHE BOOL "")
 
+# Use LLD do have less requirements on system linker, unless we're on an apple
+# platform where the system compiler is to be prefered.
+if(APPLE)
+    set(BOOTSTRAP_LLVM_ENABLE_LLD OFF CACHE BOOL "")
+else()
+    set(BOOTSTRAP_LLVM_ENABLE_LLD ON CACHE BOOL "")
+endif()
+
+
 set(CLANG_BOOTSTRAP_TARGETS
   clang
   check-all

llvm/docs/AdvancedBuilds.rst

@@ -186,8 +186,6 @@ following commands:
 .. code-block:: console
 
   $ cmake -G Ninja -C <path_to_clang>/cmake/caches/3-stage.cmake <source dir>
-  $ ninja stage3
+  $ cmake --build . --target stage3 --parallel
 
-After the build you can compare the stage2 & stage3 compilers. We have a bot
-setup `here <http://lab.llvm.org:8011/builders/clang-3stage-ubuntu>`_ that runs
-this build and compare configuration.
+After the build you can compare the stage2 & stage3 compilers.