ELF: Implement basic support for --version-script.

This patch only implements support for version scripts of the form:
  { [ global: symbol1; symbol2; [...]; symbolN; ] local: *; };
No wildcards are supported, other than for the local entry. Symbol versioning
is also not supported.

It works by introducing a new Symbol flag which tracks whether a symbol
appears in the global section of a version script.

This patch also simplifies the logic in SymbolBody::isPreemptible(), and
teaches it to handle the case where symbols with default visibility in DSOs
do not appear in the dynamic symbol table because of a version script.

Fixes PR27482.

Differential Revision: http://reviews.llvm.org/D19430

llvm-svn: 267208