[llvm-cov] Avoid 0% when reporting something that's 0/0

This commit makes llvm-cov avoid showing 0% (0/0) coverage for things
like file function coverage, etc. in reports and HTML output. This can happen
for files like headers that have macros but no functions. This commit makes
llvm-cov report - (0/0) instead.

rdar://29246480

Differential Revision: https://reviews.llvm.org/D26615

llvm-svn: 287539