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
Roll LLVM to r209595

This incorporates a change that introduces extension points in the pass
manager for peephole optimizers (such as the Souper pass).
Register the Souper pass as a peephole extension point