Enhance llvm::SourceMgr to support diagnostic ranges, the same way clang does. Enhance

the X86 asmparser to produce ranges in the one case that was annoying me, for example:

test.s:10:15: error: invalid operand for instruction
movl 0(%rax), 0(%edx)
              ^~~~~~~

It should be straight-forward to enhance filecheck, tblgen, and/or the .ll parser to use 
ranges where appropriate if someone is interested.

llvm-svn: 142106