Fix pdb-linker-module test on linux.
lld-link is apparently a symlink to lld. Our pdb writing code
uses llvm::sys::fs::getMainExecutable() and writes that value
into the PDB, which will resolve symlinks etc. But what we
really want is the exact value of argv, which matches what
the MS linker does. Fix this by just writing argv instead
of calling getMainExecutable.