[ELF] Improve error message for incompatible section flags

Previously we were not printing out the flags of the incompatible
section which made it difficult to determine what the problem was.

The error message format has been change to the following:

  error: incompatible section flags for .bar
  >>> /foo/bar/incompatible-section-flags.s.tmp.o:(.bar): 0x403
  >>> output section .bar: 0x3

Patch by Alexander Richardson.

Differential Revision: https://reviews.llvm.org/D32484

llvm-svn: 301319