Use clang's libc++ instead of GNU's libstdc++.

Currently, the primary reason we have libstdc++ as our target is because we could not risk breaking ABI-compatibility with ParseCrashReporting.

However, since crash reporting is now deprecated, moving forward we can safely target libc++ for any C++ we may write in the future.

Configurations/Common.xcconfig

@@ -13,7 +13,7 @@
 CLANG_ENABLE_OBJC_ARC = YES
 GCC_C_LANGUAGE_STANDARD = gnu11
 CLANG_CXX_LANGUAGE_STANDARD = gnu++14
-CLANG_CXX_LIBRARY = libstdc++
+CLANG_CXX_LIBRARY = libc++
 
 // Search Paths
 PARSE_DIR = $(PROJECT_DIR)