Linker script: support VERSION command.

Summary:
VERSION commands define symbol versions. The grammar of the
commnad is as follows

  VERSION { version-script-commands }

where version-script-commands is

  [ name ] { version-definitions }.

Note that we already support version-script-commands because
it is being used for version script command.

This patch is based on George's patch https://reviews.llvm.org/D23609

Reviewers: grimar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24089

llvm-svn: 280284