Add a paragraph on prefix data layout.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191219 91177308-0d34-0410-b5e6-96231b3b80d8
Handle gnu pubtypes sections:

a) Make sure we are emitting the correct section in our section labels
when we begin the module.
b) Make sure we are emitting the correct pubtypes section in the
presence of gnu pubtypes.
c) For C++ struct, union, class, and enumeration types are default
external.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191225 91177308-0d34-0410-b5e6-96231b3b80d8