[clang][Preprocessor] Replace the slow translateFile call by a new, faster isMainFile check

The commit 3c28a2dc introduced the check that checks if we're
trying to re-enter a main file when building a preamble. Unfortunately this slowed down the preamble
compilation by 80-90% in some test cases, as translateFile is really slow. This change checks
to see if the FileEntry is the main file without calling translateFile, but by using the new
isMainFile check instead. This speeds up preamble building by 1.5-2x for certain test cases that we have.

rdar://59361291

Differential Revision: https://reviews.llvm.org/D79834