compiler: mark string parameters as readonly

Strings are readonly, but the compiler doesn't always know this. Marking
them as readonly in the frontend allows the compiler to optimize based
on this knowledge.

This provides some small code size benefits. I didn't measure running
speed.

compiler/testdata/go1.20.ll

@@ -50,7 +50,7 @@ unsafe.String.throw:                              ; preds = %entry
 declare void @runtime.unsafeSlicePanic(ptr) #1
 
 ; Function Attrs: nounwind
-define hidden ptr @main.unsafeStringData(ptr %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
+define hidden ptr @main.unsafeStringData(ptr readonly %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
 entry:
   %stackalloc = alloca i8, align 1
   call void @runtime.trackPointer(ptr %s.data, ptr nonnull %stackalloc, ptr undef) #3