[PATCH] Fix PR18381 - print a minimal diagnostic rather than assert on unresolved .secidx target

[Jim Grosbach]

  The diagnostic should use consistent text with other places where we can't find the symbol and should give some indication what the user should change in the assembly source to fix the problem. "Can't resolve the '%s' symbol" seems a bit too vague.

http://llvm-reviews.chandlerc.com/D2653