We had a dedicated `StringLexer` class that pretty much just replicates
the `llvm::StringRef` interface. This patch removes the `StringLexer` in
favour of just using `llvm::StringRef`. Much of the string parsing can
be cleaned up, but I tried to keep the changes a small as possible so
just kept the logic and replaced the APIs calls. The only awkward
side-effect of this is that we have to pass a `llvm::StringRef &`
around.
There were some gaps in the API, so added two helper methods to
consume/pop off characters from the front of the StringRef (maybe those
can be added to `llvm::StringRef` in the future).
The `StringLexer` also had a roll-back mechanism used in two places.
That can be handled by just saving a copy of the `StringRef`.
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