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: https://llvm.org/svn/llvm-project/llvm/trunk@220334 91177308-0d34-0410-b5e6-96231b3b80d8
Pacify bots and simplify r220321

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220335 91177308-0d34-0410-b5e6-96231b3b80d8
Overwrite instead of adding to archives when creating them in mri scripts.

This matches the behavior of GNU ar and also makes it easier to implemnt
support for the addlib command.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220336 91177308-0d34-0410-b5e6-96231b3b80d8
IR: Remove dead code in metadata bitcode writing, NFC

No one cares how many uses each metadata value has, so don't bother
counting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220337 91177308-0d34-0410-b5e6-96231b3b80d8