ELF: Move Visibility, IsUsedInRegularObj and MustBeInDynSym flags to Symbol.

These are properties of a symbol name, rather than a particular instance
of a symbol in an object file. We can simplify the code by collecting these
properties in Symbol.

The MustBeInDynSym flag has been renamed ExportDynamic, as its semantics
have been changed to be the same as those of --dynamic-list and
--export-dynamic-symbol, which do not cause hidden symbols to be exported.

Differential Revision: http://reviews.llvm.org/D19400

llvm-svn: 267183