As it's causing some bot failures (and per request from kbarton).
This reverts commit r358543/ab70da07286e618016e78247e4a24fcb84077fda.
llvm-svn: 358546
This change is correct because the verifier requires that at most one
argument be marked 'sret'.
NFC, removes a use of AttributeList slot APIs.
llvm-svn: 300784