[Coverage] Emit a gap area after if conditions

The area immediately after the closing right-paren of an if condition
should have a count equal to the 'then' block's count. Use a gap region
to set this count, so that region highlighting for the 'then' block
remains precise.

This solves a problem we have with wrapped segments. Consider:

  1| if (false)
  2|   foo();

Without a gap area starting after the condition, the wrapped segment
from line 1 would make it look like line 2 is executed, when it's not.

rdar://35373009

llvm-svn: 317758