disallow unknown-length pointer to opaque

This also means that translate-c has to detect when a pointer to
opaque is happening, and use `*` instead of `[*]`.

See #1059

test/compile_errors.zig

@@ -1,6 +1,13 @@
 const tests = @import("tests.zig");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "unknown length pointer to opaque",
+        \\export const T = [*]@OpaqueType();
+    ,
+        ".tmp_source.zig:1:18: error: unknown-length pointer to opaque",
+    );
+
     cases.add(
         "error when evaluating return type",
         \\const Foo = struct {