Implement .seh_stackalloc and .seh_pushframe parsing.

I haven't implemented any of the ones that take registers yet. The problem is
that for x86-64 the streamer methods expect a native x86 register number (note:
%r8-%r15 want 8-15 instead of 0-7; same for %xmm8-%xmm15). I haven't figured
out exactly how I want to do that yet.

llvm-svn: 131899