The only instance where we weren't already passing a `StringRef` with a
known length to `Symbol`'s constructor is where the argument is a string
literal. Even in that case, lazy `strlen` calls don't make sense, as the
compiler can constant-evaluate the `StringRef(const char*)` constructor.
For symbols that go into the symbol table we need the length when
calculating the hash anyway. We could get away with not calling
`getName()` for local symbols, but the total contribution of `strlen` to
the run time is already below 1%, so that would just complicate the code
for a negligible benefit.
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