Update error message to use "ld.lld" instead of "lld -flavor gnu".

Dispatching based on argv[0] seems to be more convenient for users
than dispatching based on -flavor option. Currently, when a user
invoke LLD as "lld", we recommend them pass -flavor option. This
patch changes the message so that we recommend use ld.lld, ld or
lld-link instead.

Differential Revision: http://reviews.llvm.org/D22321

llvm-svn: 275448