Clean up CMakeLists.txt
Clean up CMakeLists.txt

- PASS_CXXFLAGS is the same as LLVM_CXXFLAGS
- Remove unnecessary warning flag
- Use -std=c++11 everywhere since our minimum target compilers require it

rsas/souper,rsas/souper,jubitaneja/souper,lazyparser/souper,jubitaneja/souper,chenyang78/souper,rsas/souper,rsas/souper,regehr/souper,google/souper,chubbymaggie/souper,chubbymaggie/souper,chubbymaggie/souper,jubitaneja/souper,lazyparser/souper,regehr/souper,jubitaneja/souper,chenyang78/souper,rsas/souper,jubitaneja/souper,jubitaneja/souper,regehr/souper,chenyang78/souper,chubbymaggie/souper,chenyang78/souper,regehr/souper,chenyang78/souper,regehr/souper,lazyparser/souper,lazyparser/souper,chubbymaggie/souper,google/souper,lazyparser/souper,rsas/souper,google/souper,chenyang78/souper,google/souper,google/souper