Auto merge of #78047 - khyperia:set_span_mut, r=bjorn3

Make set_span take mut self

This was a mistake in #77614

It's not a _huge_ deal, because backends can always implement this with interior mutability, but it's nice to avoid interior mutability when possible. For context, the `set_source_location` method, called alongside `set_span`, also takes `&mut self`.

r? `@eddyb`

compiler/rustc_codegen_llvm/src/builder.rs

@@ -139,7 +139,7 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         unsafe { llvm::LLVMGetInsertBlock(self.llbuilder) }
     }
 
-    fn set_span(&self, _span: Span) {}
+    fn set_span(&mut self, _span: Span) {}
 
     fn position_at_end(&mut self, llbb: &'ll BasicBlock) {
         unsafe {

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -45,7 +45,7 @@ pub trait BuilderMethods<'a, 'tcx>:
     fn build_sibling_block(&self, name: &str) -> Self;
     fn cx(&self) -> &Self::CodegenCx;
     fn llbb(&self) -> Self::BasicBlock;
-    fn set_span(&self, span: Span);
+    fn set_span(&mut self, span: Span);
 
     fn position_at_end(&mut self, llbb: Self::BasicBlock);
     fn ret_void(&mut self);