COFF: Remove DefinedSymbol::isLive() and markLive(). NFC.

Basically the concept of "liveness" is for sections (or chunks in LLD
terminology) and not for symbols. Symbols are always available or live,
or otherwise it indicates a link failure.

Previously, we had isLive() and markLive() methods for DefinedSymbol.
They are confusing methods. What they actually did is to act as a proxy
to backing section chunks. We can simplify eliminate these methods
and call section chunk's methods directly.

llvm-svn: 247869