Revert "[clang] Remove dead incremental Parser code (llvm#102450)"

This reverts commit 67cb040.
When importing headers, the Swift ClangImporter library depends on
`clang::Parser::ParseTopLevelDecl` consuming an EOF token when
`IncrementalProcessing` is set.

Author: AnthonyLatsis

clang/lib/Parse/Parser.cpp

@@ -635,6 +635,11 @@ bool Parser::ParseTopLevelDecl(DeclGroupPtrTy &Result,
                                Sema::ModuleImportState &ImportState) {
   DestroyTemplateIdAnnotationsRAIIObj CleanupRAII(*this);
 
+  // Skip over the EOF token, flagging end of previous input for incremental
+  // processing
+  if (PP.isIncrementalProcessingEnabled() && Tok.is(tok::eof))
+    ConsumeToken();
+
   Result = nullptr;
   switch (Tok.getKind()) {
   case tok::annot_pragma_unused: