Change MemoryBuffer* to MemoryBuffer& parameter to Lexer::ComputePreamble

(dropping const from the reference as MemoryBuffer is immutable already,
so const is just redundant - and while I'd personally put const
everywhere, that's not the LLVM Way (see llvm::Type for another example
of an immutable type where "const" is omitted for brevity))

Changing the pointer argument to a reference parameter makes call sites
identical between callers with unique_ptrs or raw pointers, minimizing
the churn in a pending unique_ptr migrations.