Fix clang crash when printing highlighted code in diagnostic (after #66514) (#80442)

Implements the fix proposed by Evgeny Eltsin on
https://github.com/llvm/llvm-project/pull/66514#issuecomment-1924039038.

No test case provided, since the bug is extremely sensitive to the
preprocessor
state (headers, macros, including the ones defined on command line), and
it
turned out to be non-trivial to create an isolated test.