[dsymutil][NFC} Rename thread related command line options

This makes the command line options consistent with llvm-cov and
llvm-profdata, which both use `-num-threads` and `-j`.

This also addresses the conflict reported after landing D39355.

Differential revision: https://reviews.llvm.org/D39496

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317104 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed