[Coverage] Prevent creating a redundant counter if a nested body ends with a macro.

If there were several nested statements arranged in a way that all of them
end up with the same macro, then the expansion of this macro was assigned
with all the corresponding counters of these statements.
As a result, the wrong counter value was shown for the macro in llvm-cov.

This patch fixes the issue by preventing adding a counter for an expanded
source range if it already has an assigned counter, which is expected
to come from the most specific statement.

Differential Revision: https://reviews.llvm.org/D23160

llvm-svn: 279962