header-checker: Downgrade to C++ 14

This commit switches the option to `-std=c++14` to avoid potential ODR
violations and fix the Mac build.

This commit downgrades from `-std=c++17` to `-std=c++14` because
"llvm/Support/RWMutex.h" from clang-r370808 will use `std::shared_mutex`
when `-std=c++17` is specified but LLVM/Clang prebuilts are built with a
different implementation.

Furthermore, Mac OS X 10.8 (or 10.9) does not support
`std::shared_mutex`.  Thus, there are availability errors when we are
building header-checker for Mac OS X:

    clang-r370808/include/llvm/Support/RWMutex.h:98:8:
    error: 'shared_mutex' is unavailable: introduced in macOS 10.12

Bug: 139945549
Test: OUT_DIR=out && \
      development/vndk/tools/header-checker/android/build-prebuilts.sh

Change-Id: Ic23fbdb67f7cd0700a3001c644485f7d8d0ee3e2

vndk/tools/header-checker/src/utils/string_utils.cpp

@@ -14,6 +14,8 @@
 
 #include "utils/string_utils.h"
 
+#include <llvm/ADT/Optional.h>
+
 #include <algorithm>
 #include <cctype>
 #include <cstdlib>
@@ -72,7 +74,7 @@ std::vector<std::string_view> Split(std::string_view s,
 }
 
 
-std::optional<int> ParseInt(const std::string &s) {
+llvm::Optional<int> ParseInt(const std::string &s) {
   const char *start = s.c_str();
   if (*start == '\0') {
     return {};