- fix for string conversion losing precision

Summary:
TSIL

diff is part of re-enabling -Wconversion for HSR.

```
buck-out/v2/gen/fbcode/07838712e61ba1ef/folly/__string__/buck-headers/folly/String-inl.h:222:33: error: implicit conversion loses integer precision: 'int' to 'char' [-Werror,-Wimplicit-int-conversion]
  222 |         out.push_back((h1 << 4) | h2);
```

resolved by casting the type of h1.

Reviewed By: mshamis85

Differential Revision: D71142996

fbshipit-source-id: 4b3a873a72784b2fb556d62a33c5b307bf666007

Author: Andy Firth

third-party/folly/src/folly/String-inl.h

@@ -219,7 +219,7 @@ bool tryUriUnescape(StringPiece str, String& out, UriEscapeMode mode) {
           return false;
         }
         out.append(&*last, size_t(p - last));
-        out.push_back((h1 << 4) | h2);
+        out.push_back(decltype(h1)(h1 << 4) | h2);
         p += 3;
         last = p;
         break;