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

include/llvm/Support/COFF.h

@@ -531,6 +531,8 @@ namespace COFF {
   };
 
   enum DLLCharacteristics {
+    /// ASLR with 64 bit address space.
+    IMAGE_DLL_CHARACTERISTICS_HIGH_ENTROPY_VIRTUAL_ADDRESS = 0x0020,
     /// DLL can be relocated at load time.
     IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE = 0x0040,
     /// Code integrity checks are enforced.