Add stub sections about Parse, Sema, CodeGen to the internals manual.

I'd be interested if the paragraph on Parse not knowing much about AST is
something folks agree with.  I think this used to be true after rjmccall removed
the Action interface in r112244 and I believe it's still true, but I'm not sure.
(For example, ParseOpenMP.cpp does include AST/StmtOpenMP.h.  Other than that,
Parse not using AST nodes much seems to be still true, though.)