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.

llvm-svn: 189126
GitOrigin-RevId: 1e50cbf366e9a8c0660189c34273bab22fe8b4ee