[Coverage] Emit gap areas in braces-optional statements (PR35387)

Emit a gap area starting after the r-paren location and ending at the
start of the body for the braces-optional statements (for, for-each,
while, etc). The count for the gap area equal to the body's count. This
extends the fix in r317758.

Fixes PR35387, rdar://35570345

Testing: stage2 coverage-enabled build of clang, check-clang
llvm-svn: 319373