[llvm-cov] Rearrange entries in report index.

Files which don't contain any functions are likely useless; don't
include them in the main table. Put the links at the bottom of the
page, in case someone wants to figure out coverage for code inside
a macro.

Not sure if this is the best solution, but it seems like an
improvement.

Differential Revision: https://reviews.llvm.org/D36298

llvm-svn: 310518