[llvm-cov] Create syntax to pass source w/o binary.

Since binary ID lookup makes CLI object arguments optional, it should be
possible to pass a list of source files without a binary. Unfortunately,
the current syntax will always interpret the first source file as a
binary. This change adds a `-sources` option to cause all later
positional arguments to be considered sources.

Reviewed By: gulfem

Differential Revision: https://reviews.llvm.org/D144207