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[0], which matches what
the MS linker does.  Fix this by just writing argv[0] instead
of calling getMainExecutable.

llvm-svn: 307592