message: Add two phase LLVM build to Ubuntu 16.04 docker
diff: ARG LLVM_VERSION=12.0
  -FROM ubuntu:16.04 AS llvm_build_only
  +FROM ubuntu:16.04 AS llvm_build_step1
  MAINTAINER Dmitry Babokin <dmitry.y.babokin@intel.com>
  SHELL ["/bin/bash", "-c"]
  @@ -51,13 +51,18 @@ WORKDIR /usr/local/src/ispc
  # i.e. if clang was built by gcc, you may need to use gcc to build ispc (i.e. run "make gcc"),
  # or better do clang selfbuild and use it for ispc build as well (i.e. just "make").
  # "rm" are just to keep docker image small.
  -RUN python3.6 ./alloy.py -b --version=$LLVM_VERSION --selfbuild --verbose $EXTRA_BUILD_ARG && \
  - rm -rf $LLVM_HOME/build-$LLVM_VERSION $LLVM_HOME/llvm-$LLVM_VERSION $LLVM_HOME/bin-"$LLVM_VERSION"_temp $LLVM_HOME/build-"$LLVM_VERSION"_temp
  +RUN python3.6 ./alloy.py -b --version=$LLVM_VERSION --selfbuild-phase1 --verbose $EXTRA_BUILD_ARG && \
  + rm -rf $LLVM_HOME/build-"$LLVM_VERSION"_temp
  -ENV PATH=$LLVM_HOME/bin-$LLVM_VERSION/bin:$PATH
  +FROM llvm_build_step1 AS llvm_build_step2
  +ARG LLVM_VERSION
  +ARG EXTRA_BUILD_ARG
  -FROM llvm_build_only AS ispc_build
  +RUN python3.6 ./alloy.py -b --version=$LLVM_VERSION --selfbuild-phase2 --verbose $EXTRA_BUILD_ARG && \
  + rm -rf $LLVM_HOME/build-$LLVM_VERSION $LLVM_HOME/llvm-$LLVM_VERSION $LLVM_HOME/bin-"$LLVM_VERSION"_temp
  +ENV PATH=$LLVM_HOME/bin-$LLVM_VERSION/bin:$PATH
  +FROM llvm_build_step2 AS ispc_build
  ARG LLVM_VERSION
  # ISPC builds in C++17 mode and will fail without modern libstdc++
  