[llvm-ifs][IFS] llvm Interface Stubs merging + object file generation tool.

This tool merges interface stub files to produce a merged interface stub file
or a stub library. Currently it for stub library generation it can produce an
ELF .so stub file, or a TBD file (experimental). It will be used by the clang
-emit-interface-stubs compilation pipeline to merge and assemble the per-CU
stub files into a stub library.

The new IFS format is as follows:

--- !experimental-ifs-v1
IfsVersion:      1.0
Triple:          <llvm triple>
ObjectFileFormat: <ELF | TBD>
Symbols:
  _ZSymbolName: { Type: <type>, etc... }
...

Differential Revision: https://reviews.llvm.org/D66405

llvm-svn: 370499