[clangd] Treat preamble patch as main file for include-cleaner analysis

Since we redefine all macros in preamble-patch, and it's parsed after
consuming the preamble macros, we can get false missing-include diagnostics
while a fresh preamble is being rebuilt.

This patch makes sure preamble-patch is treated same as main file for
include-cleaner purposes.

Differential Revision: https://reviews.llvm.org/D148143