Decouple MachBufferFinalized<Stencil> from ir::FunctionParameters (#5419)

This commit allows retrieving a `MachBufferFinalized<Final>` from a
`MachBufferFinalized<Stencil>` without relying on
`ir::FunctionParameters`. Instead it uses the function's base source location,
which is the only piece used by the previous `apply_params` definition.

This change allows other uses cases (e.g. Winch) to use an opaque, common
concept, exposed outside of `cranelift-codegen` to get the finalized state of
the machine buffer. This change implies that Winch will transitively know about
the `Stencil` compilation phase, but the `Stencil` phase is not exposed to Winch.

Other alternatives considered:

Parametrizing `MachBufferFinzalized` in a way such that it allows specifying
which compilation phase the caller is targetting. Such approach would require
also parametrizing the `MachSrcLoc` definition. One of the main drawbacks of
this approach is that it also requires changing how the `MachBuffer`'s
`start_srcloc` works: for caller requesting a `Final` `MachBufferFinalized`, the
`MachBuffer` will need to work in terms of `SourceLoc` rather than in
`RelSourceLoc` terms. This approach doesn't necessarily present more advantages
than the approach presented in this change, in which there's no need to make any
fundamental changes and in which all the `cranelift-codegen` primitives are
already exposed.

cranelift/codegen/src/machinst/mod.rs

@@ -323,7 +323,7 @@ impl CompiledCodeStencil {
     /// Apply function parameters to finalize a stencil into its final form.
     pub fn apply_params(self, params: &FunctionParameters) -> CompiledCode {
         CompiledCode {
-            buffer: self.buffer.apply_params(params),
+            buffer: self.buffer.apply_base_srcloc(params.base_srcloc()),
             frame_size: self.frame_size,
             disasm: self.disasm,
             value_labels_ranges: self.value_labels_ranges,