[ELF] Suggest extern "C" when the definition is mangled while an undefined reference is not

The definition may be mangled while an undefined reference is not.
This may come up when (1) the reference is from a C file or (2) the definition
misses an extern "C".

(2) is more common. Suggest an arbitrary mangled name that matches the
undefined reference, if such a definition exists.

  ld.lld: error: undefined symbol: foo
  >>> referenced by a.o:(.text+0x1)
  >>> did you mean to declare foo(int) as extern "C"?
  >>> defined in: a1.o

Reviewed By: dblaikie, ruiu

Differential Revision: https://reviews.llvm.org/D69650