std.builtin: make returnError not take the error trace

It's now unnecessary to explicitly pass this, because it receives an
implicit error trace parameter anyway, so can just use
`@errorReturnTrace()`. The previous commit updated Sema to expect this
new interface. This saves an AIR instruction at all `returnError` call
sites.

Author: mlugg

lib/std/builtin.zig

@@ -1150,9 +1150,10 @@ pub const panicInactiveUnionField = Panic.inactiveUnionField;
 /// To be deleted after zig1.wasm is updated.
 pub const panic_messages = Panic.messages;
 
-pub noinline fn returnError(st: *StackTrace) void {
+pub noinline fn returnError() void {
     @branchHint(.unlikely);
     @setRuntimeSafety(false);
+    const st = @errorReturnTrace().?;
     if (st.index < st.instruction_addresses.len)
         st.instruction_addresses[st.index] = @returnAddress();
     st.index += 1;