Support: add Windows ARM EH data structures

Introduce the support structures necessary to deal with the Windows ARM EH data.
These definitions are extremely aggressive about assertions to aid future use
for generation of the entries and subsequent decoding.

The names for the various fields are meant to reflect the names used by the
Visual Studio toolchain to aid communication.

Due to the complexity in reading a few of the values, there are a couple of
additional utility functions to decode the information.

In general, there are two ways to encode the unwinding information:
- packed, which places the data inline into the
  _IMAGE_ARM_RUNTIME_FUNCTION_ENTRY structure.
- unpacked, which places the data into auxiliary structures placed into the
  .xdata section.

The set of structures allow reading of data in either encoding, with the minor
caveat that epilogue scopes need to be decoded manually by constructing the
structure from the data returned by the RuntimeFunction structure.

These definitions are meant for read-only access at the current point as the
first use of them will be to decode the exception information.

llvm-svn: 209998