Automatically setup repo toolchain and build and run tests about llvm-prepo

[MaggieYingYi]

The task 1 (automatically setup repo toolchain) has been finished and the scripts have been committed in git 7f20b75.

Build and use the repo toolchain with CMake:

Step 1: build the compiler following the steps mentioned in https://github.com/SNSystems/llvm-prepo/blob/master/README.md. The make_build.py python script could be used to generate a build, for example:

$ cd  ~/llvm-prepo/
$ tools/pstore/utils/make_build.py

Step2: copy the repo.json (in the ~/llvm-prepo/utils/repo folder) to your pstore local repository (in the ~/llvm-prepo/tools/pstore folder), and modify the repo2obj to point to the repo2obj tool in your llvm-pstore build directory. After modifying the repo.json, the contents are:

$ cd  ~/llvm-prepo/tools/pstore
$ vim repo.cmake
{
  "ar": "/usr/bin/ar",
  "link": "/usr/bin/clang++",
  "repo2obj": "/home/username/llvm-prepo/build_linux/bin/repo2obj"
}

Step 3: generate a build using the repo.camke toolchain file. The user must specify CMAKE_C_COMPILER, CMAKE_CXX_COMPILER and utils_dir. For example, using the following command to generate a pstore build using the repo toolchain.

$ cd ~/llvm-prepo/tools/pstore
$ utils/make_build.py -DCMAKE_TOOLCHAIN_FILE=/home/username/llvm-prepo/utils/repo/repo.cmake -DCMAKE_CXX_COMPILER=/home/username/llvm-prepo/build_linux/bin/clang++ -DCMAKE_C_COMPILER=/home/username/llvm-prepo/build_linux/bin/clang -Dutils_dir=/home/username/llvm-prepo/utils/repo -o Build

[MaggieYingYi]

Finished and committed in git commits: 7f20b75 and 91591ab.