[clang] [clang][NFC] move traits to tablegen (PR #201491)

[Matthias Wippich via cfe-commits]

================
@@ -0,0 +1,556 @@
+class Trait<string P> {
+  string Prefix = P;
+  string Spelling = "";
----------------
Tsche wrote:

Since aliases can have different keyflags, a `list<string>` did not seem sufficient (especially if we want to reuse this to tablegen keywords at some point). I went with
```
class Alias<Trait P> {
  Trait Primary = P;
  string Spelling = "";
  list<TokenKey> KeyFlags = [KEYCXX];
}
```
instead. Is that fine? :)

https://github.com/llvm/llvm-project/pull/201491