Everywhere: Warn on function definitions without prototypes

If no header includes the prototype of a function, then it cannot be
used from outside the translation unit it was defined in. In that case,
it should be marked as `static`, in order to avoid possible ODR
problems, unnecessary exported symbols, and allow the compiler to better
optimize those.

If this warning triggers in a function defined in a header, `inline`
needs to be added, otherwise if the header is included in more than one
TU, it will fail to link with a duplicate definition error.

The reason this diff got so big is that Lagom-only code wasn't built
with this flag even in Serenity times.

Meta/Lagom/Tools/CodeGenerators/LibTextCodec/GenerateEncodingIndexes.cpp

@@ -15,6 +15,7 @@
 #include <LibCore/File.h>
 #include <LibMain/Main.h>
 
+namespace {
 struct LookupTable {
     u32 first_pointer;
     u32 max_code_point;
@@ -175,6 +176,7 @@ namespace TextCodec {
     TRY(file.write_until_depleted(generator.as_string_view().bytes()));
     return {};
 }
+} // end anonymous namespace
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {