GSYM: Add the llvm::gsym::Header header class with tests

This patch adds the llvm::gsym::Header class which appears at the start of a stand alone GSYM file, or in the first bytes of the GSYM data in a GSYM section within a file. Added encode and decode methods with full error handling and full tests.

Differential Revision: https://reviews.llvm.org/D67666

llvm-svn: 372149