Comment parsing: fix a bug where a line with whitespace between two paragraphs

Comment parsing: fix a bug where a line with whitespace between two paragraphs
would cause us to concatenate these paragraphs into a single one.

The no-op whitespace churn in test/Index test happened because these tests
don't use the correct approach for testing and are more strict than required
for they are testing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189126 91177308-0d34-0410-b5e6-96231b3b80d8