[clang] [clang] Avoid calling isInSystemMacro() too often (PR #182217)

[Timm Baeder via cfe-commits]

tbaederr wrote:

With this patch applied:
https://llvm-compile-time-tracker.com/compare.php?from=4fd9c1fccb0308d63bf6c18c03fd1409de7423e8&to=7a28c5b7bf7e1a5c32b5222502339756a1ce7a42&stat=instructions:u

https://github.com/llvm/llvm-project/pull/182217