minidump: Create memory regions from the sections of loaded modules

Summary:
Not all minidumps contain information about memory permissions. However,
it is still important to know which regions of memory contain
potentially executable code. This is particularly important for
unwinding on win32, as the default unwind method there relies on
scanning the stack for things which "look like" code pointers.

This patch enables ProcessMinidump to reconstruct the likely permissions
of memory regions using the sections of loaded object files. It only
does this if we don't have a better source (memory info list stream, or
linux /proc/maps) for this information, and only if the information in
the object files does not conflict with the information in the minidump.

Theoretically that last bit could be improved, since the permissions
obtained from the MemoryList streams is also only a very rough guess,
but it did not seem worthwhile to complicate the implementation because
of that because there will generally be no ov...