[clangd] Workaround the comments crash, reenable the test.

This fix is still quite fragile, the underlying problem is that the
code should not rely on source ranges coming from the preamble to be
correct when reading from the text buffers.
This is probably not possible to achieve in practice, so we would
probably have to keep the contents of old headers around for the
lifetime of the preamble.

llvm-svn: 333369