Comment necessity of early initialization

Code review feedback from Eric Christopher on r204261.

I didn't want to go into too much detail (the revision history should
provide the full stuff) - but I can add more if that's preferred.

Also moved this up to right by the construction of the MCAsmInfo so
there's less chance that other things might sneak in in between.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205267 91177308-0d34-0410-b5e6-96231b3b80d8