Revert "[Parse] Always end translation unit"

This reverts commit 76cfe6c.

Only running `ActOnEndOfTranslationUnit` when
`enableIncrementalProcessing` was false was the only NFC part of
splitting `-fincremental-extensions` and `enableIncrementalProcessing`
again (68090ee). LLDB started failing
after that change with:
```
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/cstdlib:127:9: error: declaration of 'calloc' must be imported from module 'Darwin.malloc._malloc' before it is required
```

Reverting that part of the change fixed LLDB and didn't cause any other
test failures, so happy days.

Unfortunately most of what `Sema::ActOnEndOfTranslationUnit` does is
additional diagnostics, which don't make a lot of sense from the Clang
Importer in Swift. So this ends up being a significant regression if eg.
`-warnings-as-errors` is enabled.

Resolves rdar://116194950.

(cherry picked from commit eed23a1)

Author: bnbarham

clang/lib/Parse/Parser.cpp

@@ -730,7 +730,8 @@ bool Parser::ParseTopLevelDecl(DeclGroupPtrTy &Result,
 
     // Late template parsing can begin.
     Actions.SetLateTemplateParser(LateTemplateParserCallback, nullptr, this);
-    Actions.ActOnEndOfTranslationUnit();
+    if (!PP.isIncrementalProcessingEnabled())
+      Actions.ActOnEndOfTranslationUnit();
     //else don't tell Sema that we ended parsing: more input might come.
     return true;