[clang/Lexer] Enhance `Lexer::getImmediateMacroNameForDiagnostics` to return a...

[clang/Lexer] Enhance `Lexer::getImmediateMacroNameForDiagnostics` to return a result from non-file buffers

Use `SourceManager::isWrittenInScratchSpace()` to specifically check for token paste or stringization, instead of
excluding all non-file buffers. This allows diagnostics to mention macro names that were defined from the command-line.

Differential Revision: https://reviews.llvm.org/D140164