Frontend: Clarify logic for using the preamble in ASTUnit::CodeComplete, almost NFC

Clarify the logic for using the preamble (and overriding the main file
buffer) in `ASTUnit::CodeComplete` by factoring out a couple of lambdas
(`getUniqueID` and `hasSameUniqueID`).  While refactoring the logic,
hoist the check for `Line > 1` and locally check if the filenames are
equal (both to avoid unnecessary `stat` calls) and skip copying out the
filenames to `std::string`.

Besides fewer calls to `stat`, there's no functionality change here.

Differential Revision: https://reviews.llvm.org/D91296