Fix the partial backtrace when using a combination of stripped function symbols

and -fomit-frame-pointer.

- Parses eh_frame FDEs to determine the function address and size so that
the call frame parsing can continue.

Note: This code path is specific to ELF and PECOFF, because ObjectFileMachO
uses LCT_FunctionStarts to efficiently populate the symbol table.

Thanks to Jason Molenda for the review!

llvm-svn: 186585