Rename IMAGE_DLL_CHARACTERISTICS_HIGH_ENTROPY_VA.

editbin.exe and link.exe both accepts /highentropyva option to set this bit, so
doing s/VIRTUAL_ADDRESS/VA/ should make sense.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200191 91177308-0d34-0410-b5e6-96231b3b80d8