[llvm-tapi] initial commit, supports ELF text stubs

http://lists.llvm.org/pipermail/llvm-dev/2018-September/126472.html

TextAPI is a library and accompanying tool that allows conversion between binary shared object stubs and textual counterparts. The motivations and uses cases for this are explained thoroughly in the llvm-dev proposal [1]. This initial commit proposes a potential structure for the TAPI library, also including support for reading/writing text-based ELF stubs (.tbe) in addition to preliminary support for reading binary ELF files. The goal for this patch is to ensure the project architecture appropriately welcomes integration of Mach-O stubbing from Apple's TAPI [2].

Added:

 - TextAPI library
 - .tbe read support
 - .tbe write (to raw_ostream) support

[1] http://lists.llvm.org/pipermail/llvm-dev/2018-September/126472.html
[2] https://github.com/ributzka/tapi

Differential Revision: https://reviews.llvm.org/D53051

llvm-svn: 348170