message: Use the newly added VERSION.llvm.svn in the instructions about building MLIR
diff: @@ -102,8 +102,9 @@ gcc 7.
  ```sh
  git clone https://github.com/llvm/llvm-project.git
  git clone https://github.com/tensorflow/mlir llvm-project/llvm/projects/mlir
  -mkdir llvm-project/build
  -cd llvm-project/build
  +cd llvm-project/
  +git checkout $(git log --grep=$(cat projects/mlir/VERSION.llvm.svn) --pretty="format:%H" -n 1)
  +mkdir build && build
  cmake -G Ninja ../llvm -DLLVM_BUILD_EXAMPLES=ON -DLLVM_TARGETS_TO_BUILD="host"
  cmake --build . --target check-mlir
  ```
  @@ -116,8 +117,14 @@ REM $visual-studio-install\Auxiliary\Build\vcvarsall.bat" x64
  REM invoked.
  git clone https://github.com/llvm/llvm-project.git
  git clone https://github.com/tensorflow/mlir llvm-project\llvm\projects\mlir
  -mkdir llvm-project\build
  -cd llvm-project\build
  +cd llvm-project
  +cat projects/mlir/VERSION.llvm.svn
  +# replace {SVN_VERSION} with the result of the previous command
  +git log --grep={SVN_VERSION} --pretty="format:%H" -n 1
  +# replace {GIT_HASH} with the result of the previous command
  +git checkout {GIT_HASH}
  +mkdir build
  +cd build
  cmake ..\llvm -G "Visual Studio 15 2017 Win64" -DLLVM_BUILD_EXAMPLES=ON -DLLVM_TARGETS_TO_BUILD="host" -DCMAKE_BUILD_TYPE=Release -Thost=x64
  cmake --build . --target check-mlir
  ```
  