Improve the handling of missing elf symtab and missing symbol sizes

* Generate artificial symbol names from eh_fame during symbol parsing
  so these symbols are already present when we calcualte the size of
  the symbols where 0 is specified.
* Fix symbol size calculation for the last symbol in the file where
  it have to last until the end of the parent section.

This is the re-commit of the original change after fixing some test
failures on OSX.

Differential revision: http://reviews.llvm.org/D16996

llvm-svn: 261205