Convert a few std::string with StringRef. NFC.
Convert a few std::string with StringRef. NFC.

This is a micro optimization, but also makes the code a bit more flexible.

The MRIMembers variable is a short term hack. It is going away in the next
commit.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@220334 91177308-0d34-0410-b5e6-96231b3b80d8

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