[clang] [clang-tools-extra] Remove `StringLiteral` in favor of `StringRef` (PR #122366)

[Benoit Jacob via cfe-commits]

================
@@ -57,7 +57,7 @@ class DeclarationFragments {
     Keyword,
     Attribute,
     NumberLiteral,
-    StringLiteral,
+    StringRef,
----------------
bjacob wrote:

Thanks for the heads up -- will fix one by one.

Also, note to self:  let's leep `StringLiteral` around as deprecated to give downstreams a smoother transition.

https://github.com/llvm/llvm-project/pull/122366