[Driver] Use ld.lld directly for Fuchsia rather than passing flavor

Passing a flavor to LLD requires command line argument, but if these
are being passed through a response file, this will fail because LLD
needs to know which driver to use before processing the response file.
Use ld.lld directly instead to avoid this issue.

Differential Revision: https://reviews.llvm.org/D39176

llvm-svn: 316379