[Vectorizer] Require C++17 for early-exit tests (#365)

std::reduce was introduced in C++17.

Author: huntergr-arm

SingleSource/UnitTests/Vectorizer/CMakeLists.txt

@@ -1,5 +1,6 @@
 llvm_singlesource()
 set_property(TARGET runtime-checks PROPERTY CXX_STANDARD 17)
+set_property(TARGET early-exit PROPERTY CXX_STANDARD 17)
 
 # The VPlan-native path is specific to llvm.
 if ("${CMAKE_C_COMPILER_ID}" MATCHES "Clang")