Documentation cleanup: Doxygen-ification, typo fixes, and changing some of
the duplicated documentation from .cpp files so that it's not processed by
Doxygen and hence doesn't generate duplicate output.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195799 91177308-0d34-0410-b5e6-96231b3b80d8