[clangd] Copy existing includes in ReplayPreamble
ReplayPreamble was just grabbing the reference of IncludeStructure
passed to it and then replayed any includes seen so while exiting
This implies any include seen in built-in files being replayed as part
of preamble, even though they are not. This wasn't an issue until we've
started patching preambles, as includes from built-in files were not
mapped back to main-file.
This patch copies over existing includes at the time of
ReplayPreamble::attach and only replies those to prevent any includes
from the preamble patch getting mixed-in.