[llvm-cov] Minor cleanups to prepare for the html format patch

- Add renderView{Header,Footer}, renderLineSuffix, and hasSubViews to
  support creating tables with nested views.

- Move the 'Format' cl::opt to make it easier to extend.

- Just create one function view file, instead of overwriting the same
  file for every new function. Add a regression test for this.

llvm-svn: 274086