typecheck the panic function

this adds the prototype of panic to @import("builtin")
and then uses it to do an implicit cast of the panic
function to this prototype, rather than redoing all the
implicit cast logic.

closes #1894
closes #1895

src/codegen.cpp

@@ -7144,6 +7144,8 @@ Buf *codegen_generate_builtin_source(CodeGen *g) {
         "    instruction_addresses: []usize,\n"
         "};\n\n");
 
+    buf_append_str(contents, "pub const PanicFn = fn([]const u8, ?*StackTrace) noreturn;\n\n");
+
     const char *cur_os = nullptr;
     {
         buf_appendf(contents, "pub const Os = enum {\n");
@@ -7913,6 +7915,8 @@ static void gen_root_source(CodeGen *g) {
         }
     }
 
+    typecheck_panic_fn(g, g->panic_tld_fn, g->panic_fn);
+
     report_errors_and_maybe_exit(g);
 
 }