IR: Allow GenericDebugNode construction from MDString

Allow `GenericDebugNode` construction directly from `MDString`, rather
than requiring `StringRef`s.  I've refactored the `StringRef`
constructors to use these.  There's no real functionality change here,
except for exposing the lower-level API.

The purpose of this is to simplify construction of string operands when
reading bitcode.  It's unnecessarily indirect to parse an `MDString` ID,
lookup the `MDString` in the bitcode reader list, get the `StringRef`
out of that, and then have `GenericDebugNode::getImpl()` use
`MDString::get()` to acquire the original `MDString`.  Instead, this
allows the bitcode reader to directly pass in the `MDString`.

llvm-svn: 227848