COFF: Add a missing enum value for high entropy ASLR.

That bit is not documented in the PE/COFF spec published by Microsoft, so we
don't know the official name of it. I named this bit
IMAGE_DLL_CHARACTERISTICS_HIGH_ENTROPY_VIRTUAL_ADDRESS because the bit is
reported as "high entropy virtual address" by dumpbin.exe,

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200121 91177308-0d34-0410-b5e6-96231b3b80d8