[SE-0458] Disambiguate "unsafe" expression within string interpolation

String interpolation uses an end-of-file token, which we weren't
checking for. Fixes rdar://146493296

test/Unsafe/safe.swift

@@ -207,6 +207,8 @@ func moreUnsafeFunc(unsafe: [Int]) {
   // expected-warning@-1{{no unsafe operations occur within 'unsafe' expression}}
 
   _ = unsafe[1]
+
+  _ = "\(unsafe)"
 }
 
 func yetMoreUnsafeFunc(unsafe: () -> Void) {