Fix poppler benchmark. (#22616)

std::basic_string<unsigned short> will no longer work with LLVM 19.

Author: brendandahl

test/third_party/poppler/cpp/poppler-global.h

@@ -79,7 +79,7 @@ typedef unsigned int /* time_t */ time_type;
 #pragma warning(push)
 #pragma warning(disable: 4251) /* class 'A' needs to have dll interface for to be used by clients of class 'B'. */
 #endif
-class POPPLER_CPP_EXPORT ustring : public std::basic_string<unsigned short>
+class POPPLER_CPP_EXPORT ustring : public std::basic_string<char16_t>
 {
 public:
     ustring();

test/third_party/poppler/readme.txt

@@ -2,4 +2,5 @@ This is Poppler. See README and COPYING.
 
 Changes for Emscripten:
   Object.h, Array.cc, Dict.cc are modified to avoid uninitialization errors (search for 'Emscripten')
-
+  poppler-global.h is modified to use std::basic_string<char16_t> to compile with newer versions of
+    LLVM (this matches newer upstream versions of poppler)