[analyzer] Don't include Clang headers inside a namespace.

Found by Gabor Kozar!
CMake: Fix out-of-source build's symlinks.

Symlinks to clang should go in Clang's build directory, not LLVM's.
scan-build: Set CC and CXX as make variables when wrapping make builds.

Variables set in a makefile are not overridden by environment variables.
Make sure we actually override CC and CXX when using scan-build.

Patch by Steve McCoy!