[llvm-cov] Allow hiding instantiation/region coverage from summary tables

Region coverage is difficult to explain without going deep into how
coverage is implemented. Instantiation coverage is easier to explain,
but probably not useful in most cases (templates don't exist in C, and
most C++ code contains relatively few templates).

This patch adds the options "-show-region-summary" and
"-show-instantiation-summary" to allow hiding those columns.
"-show-instantiation-summary" is turned off by default.

llvm-svn: 312969