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

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.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224894 91177308-0d34-0410-b5e6-96231b3b80d8