[llvm-cov] Create directory structure when filtering using -name*= options

Before this change using any of the -name*= command line options with an output
directory would result in a single file (functions.txt/functions.html)
containing the coverage for those specific functions. Now you get the same
directory structure as when not using any -name*= options.

Differential Revision: https://reviews.llvm.org/D38280

llvm-svn: 314310