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