have AsmMatcherEmitter.cpp produce the hunk of code that gets included

have AsmMatcherEmitter.cpp produce the hunk of code that gets included
into the middle of the class, and rework how the different sections of
the generated file are conditionally included for simplicity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113163 91177308-0d34-0410-b5e6-96231b3b80d8