[ELF] - Do not allow to mix global symbols versions.

For next version script:
VER1{
  global:
  a;
};

VER2{
  global:
  a;
};
gold would produce warning like:
"warning: using 'VER1' as version for 'a' which is also named in version 'VER2' in script."

Documentation also says we do not want this duplications (https://people.freebsd.org/~deischen/symver/library_versioning.txt):
"Note that you do not want to duplicate symbols in the map file. The .symver directives are all that is required to add compatibility
symbols into old versions."

This patch restricts such mixing and makes lld to produce error in this case.

Differential revision: http://reviews.llvm.org/D21555

llvm-svn: 273396