add an explicit class for holding llvm::SourceMgr diagnostics and use

it to print them.  This gives us column numbers in the diag line.  Before:

t.s:4: error: unexpected token in argument list
 mov %eax %edx
          ^

now:
t.s:4:11: error: unexpected token in argument list
 mov %eax %edx
          ^

llvm-svn: 74732