Remove `speculative` parameter from `instantiate_poly_trait_ref_inner`

[compiler-errors]

instantiate_poly_trait_ref_inner has a speculative parameter:

rust/compiler/rustc_hir_analysis/src/astconv/mod.rs

Line 684 in f525bb4

speculative: bool,

This parameter is used to suppress several errors downstream, and has only one usage:

rust/compiler/rustc_hir_analysis/src/lib.rs

Line 285 in f525bb4

true,

... which is inside of hir_trait_to_predicates, which itself has only one usage:

rust/compiler/rustc_privacy/src/lib.rs

Lines 1215 to 1222 in f525bb4

	// The traits' privacy in bodies is already checked as a part of trait object types.
	let bounds = rustc_hir_analysis::hir_trait_to_predicates(
	self.tcx,
	trait_ref,
	// NOTE: This isn't really right, but the actual type doesn't matter here. It's
	// just required by `ty::TraitRef`.
	self.tcx.types.never,
	);

The reason why speculative = true at this call-site is because we're passing the wrong self type when instantiating the trait ref:

rust/compiler/rustc_privacy/src/lib.rs

Lines 1219 to 1221 in f525bb4

	// NOTE: This isn't really right, but the actual type doesn't matter here. It's
	// just required by `ty::TraitRef`.
	self.tcx.types.never,

If we were able to pass the correct self type here, then we can totally remove the speculative parameter. I think that would be a nice clean-up if possible.

[compiler-errors]

The reason I marked this as "hard" is that it's not immediately clear how we're gonna funnel an appropriate self type to visit_trait_ref in the TypePrivacyVisitor. The HIR tree doesn't make it easy. This work will need lots of investigation and experimentation, and may not work out in the end.