Update comments to use unreachable instead of llvm.trap, as implemented now

win64: Call __chkstk through a register with the large code model

Fixes half of PR18582. True dynamic allocas will still have a
CALL64pcrel32 which will fail.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D7267

llvm-svn: 227503