Sema: add error for non-comptime param in comptime func

Adds error for taking a non comptime parameter in a function returning a
comptime-only type but not when that type is dependent on a parameter.

Co-authored-by: Veikka Tuominen <git@vexu.eu>

test/cases/compile_errors/explain_why_fn_is_called_at_comptime.zig

@@ -4,12 +4,12 @@ const S = struct {
 };
 fn bar() void {}
 
-fn foo(a: u8) S {
-    return .{ .fnPtr = bar, .a = a };
+fn foo(comptime a: *u8) S {
+    return .{ .fnPtr = bar, .a = a.* };
 }
 pub export fn entry() void {
     var a: u8 = 1;
-    _ = foo(a);
+    _ = foo(&a);
 }
 
 // error
@@ -18,6 +18,6 @@ pub export fn entry() void {
 //
 // :12:13: error: unable to resolve comptime value
 // :12:13: note: argument to function being called at comptime must be comptime known
-// :7:15: note: function is being called at comptime because it returns a comptime only type 'tmp.S'
+// :7:25: note: function is being called at comptime because it returns a comptime only type 'tmp.S'
 // :2:12: note: struct requires comptime because of this field
 // :2:12: note: use '*const fn() void' for a function pointer type

test/cases/compile_errors/non_comptime_param_in_comptime_function.zig

@@ -0,0 +1,36 @@
+fn F(val: anytype) type {
+    _ = val;
+    return struct {};
+}
+export fn entry() void {
+    _ = F(void{});
+}
+const S = struct {
+    foo: fn () void,
+};
+fn bar(_: u32) S {
+    return undefined;
+}
+export fn entry1() void {
+    _ = bar();
+}
+// prioritize other return type errors
+fn foo(a: u32) callconv(.C) comptime_int {
+    return a;
+}
+export fn entry2() void {
+    _ = foo(1);
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:20: error: function with comptime only return type 'type' requires all parameters to be comptime
+// :1:20: note: types are not available at runtime
+// :1:6: note: param 'val' is required to be comptime
+// :11:16: error: function with comptime only return type 'tmp.S' requires all parameters to be comptime
+// :9:10: note: struct requires comptime because of this field
+// :9:10: note: use '*const fn() void' for a function pointer type
+// :11:8: note: param is required to be comptime
+// :18:29: error: return type 'comptime_int' not allowed in function with calling convention 'C'