MCParser: introduce Note and use it for ARM AsmParser

Introduce a new virtual method Note into the AsmParser.  This completements the
existing Warning and Error methods.  Use the new method to clean up the output
of the unwind routines in the ARM AsmParser.
ARM IAS: improve .eabi_attribute handling

Parse tag names as well as expressions.  The former is part of the
specification, the latter is for improved compatibility with the GNU assembler.
Fix attribute value handling to be comformant to the specification.
ARM Streamer: print out tag names

If using verbose asm, print out the friendly name of the tag if possible.
ARM IAS: refactor unwind context

Move the unwinding context for the ARM IAS into a helper class.  This is purely
a structural refactoring.  A follow up change allows for recording additional
depth to improve diagnostics.
ARM IAS: allow more depth in contextual diagnostics

Switch the context to be SmallVectors.  This allows for saving additional
context when providing previous emission sites.