[buildbot] Force lld for ThinLTO whole program devirtualization worker

[tejohnson]

Summary

Force use of lld, so that the configure step uses it. Otherwise we get
an error on the 3rd stage configure which will use the lld-only
--lto-whole-program-devirtualization option.

See: http://lab.llvm.org:8014/#/builders/137/builds/1921/steps/17/logs/stdio

[mtrofin]

lgtm

buildbot/osuosl/master/config/builders.py

# LTO and ThinLTO builders.
     'builddir': "clang-with-thin-lto-wpd-ubuntu",
     'factory' : ClangLTOBuilder.getClangWithLTOBuildFactory(
                     jobs=72,
                     lto='thin',
                     extra_configure_args_lto_stage=[
                         '-DLLVM_CCACHE_BUILD=ON',
                         '-DCMAKE_CXX_FLAGS=-O3 -Xclang -fwhole-program-vtables -fno-split-lto-unit',
                         '-DCMAKE_C_FLAGS=-O3 -Xclang -fwhole-program-vtables -fno-split-lto-unit',
-                        '-DCMAKE_EXE_LINKER_FLAGS=-Wl,--lto-whole-program-visibility'])},
+                        '-DCMAKE_EXE_LINKER_FLAGS=-Wl,--lto-whole-program-visibility -fuse-ld=lld'])},
 
     {'name' : "clang-with-lto-ubuntu",
     'tags'  : ["clang","lld","LTO"],
     'workernames' : ["as-worker-91"],
     'builddir': "clang-with-lto-ubuntu",
     'factory' : ClangLTOBuilder.getClangWithLTOBuildFactory(jobs=72)},
 
 # Builders for MLIR.