[libc] Add comparison specialization for cpp:string type

[jhuber6]

Summary

We compare this type in the string_test. It had no specialization here
so it could cause linker errors. This patch simply extends the interface
to support it.

libc/test/UnitTest/LibcTest.cpp

template bool test<__llvm_libc::cpp::UInt<320>>(
     __llvm_libc::cpp::UInt<320> RHS, const char *LHSStr, const char *RHSStr,
     const char *File, unsigned long Line);
 
 template bool test<__llvm_libc::cpp::string_view>(
     RunContext *Ctx, TestCondition Cond, __llvm_libc::cpp::string_view LHS,
     __llvm_libc::cpp::string_view RHS, const char *LHSStr, const char *RHSStr,
     const char *File, unsigned long Line);
 
+template bool test<__llvm_libc::cpp::string>(
+    RunContext *Ctx, TestCondition Cond, __llvm_libc::cpp::string LHS,
+    __llvm_libc::cpp::string RHS, const char *LHSStr, const char *RHSStr,
+    const char *File, unsigned long Line);
+
 } // namespace internal
 
 bool Test::testStrEq(const char *LHS, const char *RHS, const char *LHSStr,
                      const char *RHSStr, const char *File, unsigned long Line) {
   return internal::test(Ctx, Cond_EQ, LHS ? cpp::string(LHS) : cpp::string(),
                         RHS ? cpp::string(RHS) : cpp::string(), LHSStr, RHSStr,
                         File, Line);
 }