add option to build with clang

when set BUILD_WITH_CLANG=true, source build.sh will build with clang.

Ex.

```
export LLVM_DIR=/usr/src/clang-r487747
export LLVM_CONFIG=$LLVM_DIR/bin/llvm-config

BUILD_WITH_CLANG=true source build.sh
```

Author: joeyjiaojg

CMakeLists.txt

@@ -5,6 +5,10 @@ project("SVF")
 configure_file(${PROJECT_SOURCE_DIR}/.config.in
                ${PROJECT_BINARY_DIR}/include/Util/config.h)
 
+if(BUILD_WITH_CLANG)
+  set(CMAKE_CXX_COMPILER "${LLVM_DIR}/bin/clang++")
+endif()
+
 # We need to match the build environment for LLVM: In particular, we need C++14
 # and the -fno-rtti flag
 set(CMAKE_CXX_STANDARD 14)

build.sh

@@ -224,6 +224,8 @@ mkdir "${BUILD_DIR}"
 cmake -D CMAKE_BUILD_TYPE:STRING="${BUILD_TYPE}" \
     -DSVF_ENABLE_ASSERTIONS:BOOL=true            \
     -DSVF_SANITIZE="${SVF_SANITIZER}"            \
+    -DBUILD_WITH_CLANG:BOOL=${BUILD_WITH_CLANG} \
+    -DLLVM_DIR=${LLVM_DIR} \
     -S "${SVFHOME}" -B "${BUILD_DIR}"
 cmake --build "${BUILD_DIR}" -j ${jobs}