[clang][ifs] New interface stubs format (llvm triple based).

After posting llvm-ifs on phabricator, I made some progress in hardening up how
I think the format for Interface Stubs should look. There are a number of
things I think the TBE format was missing (no endianness, no info about the
Object Format because it assumes ELF), so I have added those and broken off
from being as similar to the TBE schema. In a subsequent commit I can drop the
other formats.

An example of how The format will look is as follows:

--- !experimental-ifs-v1
IfsVersion: 1.0
Triple: x86_64-unknown-linux-gnu
ObjectFileFormat: ELF
Symbols:
  _Z9nothiddenv: { Type: Func }
  _Z10cmdVisiblev: { Type: Func }
...

The format is still marked experimental.

Differential Revision: https://reviews.llvm.org/D66446

llvm-svn: 369715