Use starts_with instead of startswith in split-file.cpp

[QuantumSegfault]

StringRef::startswith was deprecated in LLVM 18, and removed in LLVM 19.

It's replacement is StringRef::starts_with (a simple rename/alias), introduced in LLVM 16.

[QuantumSegfault]

Requesting review
@adamdruppe

This change is enough to allow successful building of LDC with LLVM 19, while still supporting LLVM 17.

I haven't actually tried building with LLVM 17 locally though (I'm doing this to avoid having to install yet another version of LLVM). We'll see if CI passes.

[adamdruppe]

omg if it actually works i might be willing to update the llvm, i just didn't even want to fight with it.

[QuantumSegfault]

Yeah. I still need to make sure the resulting ldc2 is usable.

They sometimes do things like #189416 which causes runtime breakage without compile-time breakage. So even if LDC builds, it may not be usable. We'll see.