compiler: fix branch regressions

* getOwnedFunctionIndex no longer checks if the value is actually a
   function.
 * The callsites to `intern` that I added want to avoid the `getCoerced`
   call, so I added `intern2`.
 * Adding to inferred error sets should not happen if the destination
   error set is not the inferred error set of the current Sema instance.
 * adhoc_inferred_error_set_type can be seen by the backend. Treat it
   like anyerror.

src/Module.zig

@@ -751,6 +751,7 @@ pub const Decl = struct {
         };
     }
 
+    /// This returns an InternPool.Index even when the value is not a function.
     pub fn getOwnedFunctionIndex(decl: Decl) InternPool.Index {
         return if (decl.owns_tv) decl.val.toIntern() else .none;
     }
@@ -4978,7 +4979,7 @@ fn scanDecl(iter: *ScanDeclIter, decl_sub_index: usize, flags: u4) Allocator.Err
     decl.has_align = has_align;
     decl.has_linksection_or_addrspace = has_linksection_or_addrspace;
     decl.zir_decl_index = @as(u32, @intCast(decl_sub_index));
-    if (decl.getOwnedFunctionIndex() != .none) {
+    if (decl.getOwnedFunction(mod) != null) {
         switch (comp.bin_file.tag) {
             .coff, .elf, .macho, .plan9 => {
                 // TODO Look into detecting when this would be unnecessary by storing enough state