[Coverage] Suppress creating a code region if the same area is covered by an expansion region.

In most cases these code regions are just redundant, but sometimes they
could be assigned to the counter of the parent code region instead of
the counter of the nested block.

Differential Revision: https://reviews.llvm.org/D23987


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280199 91177308-0d34-0410-b5e6-96231b3b80d8