[llvm-readobj][COFF] Print load config GuardFlags as enum flags

Print flags as documented in MS docs.
https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#load-configuration-layout
https://docs.microsoft.com/en-us/windows/win32/secbp/pe-metadata

EH_CONTINUATION_TABLE_PRESENT is not mentioned in the docs but is
instead taken from Windows SDK headers.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D132823