MC: rename Win64EHFrameInfo to WinEH::FrameInfo

The frame information stored in this structure is driven by the requirements for
Windows NT unwinding rather than Windows 64 specifically.  As a result, this
type can be shared across multiple architectures (ARM, AXP, MIPS, PPC, SH).
Rename this class in preparation for adding support for supporting unwinding
information for Windows on ARM.

Take the opportunity to constify the members as everything except the
ChainedParent is read-only.  This required some adjustment to the label
handling.

llvm-svn: 214663