[BOLT] Upgrade to C++17

[kazu]

Summary

Without this patch, I am getting errors like:

llvm-project/llvm/include/llvm/ADT/StringRef.h:233:7: error: use of
the 'nodiscard' attribute is a C++17 extension
[-Werror,-Wc++17-extensions]

bolt/CMakeLists.txt

 include(ExternalProject)
 
 set(BOLT_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(BOLT_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 
 set(BOLT_ENABLE_RUNTIME OFF)
 if (CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64")
   set(BOLT_ENABLE_RUNTIME ON)
 endif()
 
 set(BOLT_CLANG_EXE "" CACHE FILEPATH "Path to clang executable for the target \
 architecture for use in BOLT tests")