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

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
          ^

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74732 91177308-0d34-0410-b5e6-96231b3b80d8