[CMake] Set the C++ standard to C++14

Swift inherited C++14 when LLVM made the switch. However, the C++
standard is not set in the Swift project itself. This lead to compile
errors for files that include C++11 incompatible code.

Author: JDevlieghere

CMakeLists.txt

@@ -27,6 +27,11 @@ endif()
 
 ENABLE_LANGUAGE(C)
 
+# Use C++14.
+set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+set(CMAKE_CXX_STANDARD_REQUIRED YES)
+set(CMAKE_CXX_EXTENSIONS NO)
+
 # First include general CMake utilities.
 include(SwiftUtils)
 include(CheckSymbolExists)