COFF: Add /nosymtab command line option.

This is an LLD extension to MSVC link.exe command line. MSVC linker
does not write symbol tables for executables. We do unless no /debug
option is given.

There's a situation that we want to enable debug info but don't want
to emit the symbol table. One example is when we are comparing output
file size. With this patch, you can tell the linker to not create
a symbol table by just specifying /nosymtab.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@248225 91177308-0d34-0410-b5e6-96231b3b80d8