Introduce CommonSymbol.

Previously, we handled common symbols as a kind of Defined symbol,
but what we were doing for common symbols is pretty different from
regular defined symbols.

Common symbol and defined symbol are probably as different as shared
symbol and defined symbols are different.

This patch introduces CommonSymbol to represent common symbols.
After symbols are resolved, they are converted to Defined symbols
residing in a .bss section.

Differential Revision: https://reviews.llvm.org/D61895

llvm-svn: 360841