[ELF] Add a spell corrector for "undefined symbol" diagnostics

Non-undefined symbols with Levenshtein distance 1 or a transposition are
suggestion candidates. This is probably good enough and it can suggest
some missing/superfluous qualifiers: const, restrict, volatile, & and &&
ref-qualifier, e.g.

   error: undefined symbol: foo(int*)
   >>> referenced by b.o:(.text+0x1)
  +>>> did you mean: foo(int const*)
  +>>> defined in: a.o

   error: undefined symbol: foo(int*&)
   >>> referenced by b.o:(.text+0x1)
  +>>> did you mean: foo(int*)
  +>>> defined in: b.o

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D67039

llvm-svn: 370853