[lld-link] Generalize handling of /debug and /debug:{none,full,fastlink,ghash,symtab}

Implement final argument precedence if multiple /debug arguments are passed on the command-line to match expected link.exe behavior.
Support /debug:none and emit warning for /debug:fastlink with automatic fallback to /debug:full.
Emit error if last /debug:option is unknown.
Emit warning if last /debugtype:option is unknown.

https://reviews.llvm.org/D50404

llvm-svn: 342894