[clangd] Copy existing includes in ReplayPreamble

Summary:
ReplayPreamble was just grabbing the reference of IncludeStructure
passed to it and then replayed any includes seen so while exiting
built-in file.

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.

Reviewers: sammccall, jkorous

Subscribers: ilya-biryukov, MaskRay, dexonsmith, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80988