Move alias symbols from ELFLinkingContext to LinkingContext.

Alias symbols are SimpleDefinedAtoms and are platform neutral. They
don't have to belong ELF. This patch is to make it available to all
platforms. No functionality change intended.

Differential Revision: http://reviews.llvm.org/D3862

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209475 91177308-0d34-0410-b5e6-96231b3b80d8