Fix format errors

[morganchen12]

The original errors are here:

Firestore/core/src/api/expressions.h:46:  "virtual" is redundant since function is already declared as "override"  [readability/inheritance] [4]
Firestore/core/src/api/pipeline_result.h:54:  Single-parameter constructors should be marked explicit.  [runtime/explicit] [5]
Firestore/core/src/api/query_snapshot.cc:93:  Add #include <vector> for vector<>  [build/include_what_you_use] [4]
Firestore/core/src/api/query_snapshot.h:69:  Add #include <vector> for vector<>  [build/include_what_you_use] [4]
Firestore/core/src/api/realtime_pipeline_snapshot.h:71:  #endif line should be "#endif  // FIRESTORE_CORE_SRC_API_REALTIME_PIPELINE_SNAPSHOT_H_"  [build/header_guard] [5]
Firestore/core/src/core/view.cc:26:  Include the directory when naming .h files  [build/include_subdir] [4]

I'm unsure how to fix this error:

Error: Directories 'Firestore/Source/Public/FirebaseFirestore/' and 'FirebaseFirestoreInternal/FirebaseFirestore/' differ:
Files only in 'Firestore/Source/Public/FirebaseFirestore/':
- FIRPipelineBridge.h
Files only in 'FirebaseFirestoreInternal/FirebaseFirestore/':

Copying the file over seemed to break a bunch of stuff.

#no-changelog

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[cherylEnkidu]

I can create a PR to fix the error on C++ side. For the objective-C one, I remember @NickChittle mentioned one of the directory is auto-generated. But I don't know how to generate those.

[cherylEnkidu]

Actually, I am going to change on your branch to avoid so much rebase ...

[morganchen12]

Sounds good, feel free to take over the PR

[ncooke3]

Discussed with Cheryl offline. We agreed to delete this check.

It's original purpose was to ensure that the FirebaseFirestoreInternal headers intended for public use were re-exported under the FirebaseFirestore namespace. This was for backwards compatibility, so existing ObjC client code didn't break when we refactored Firestore into FirebaseFirestore (Swift) and FirebaseFirestoreInternal (ObjC/C++). We have reached a point where new headers are only public so FirebaseFirestore (Swift) can use them and not for downstream ObjC clients to use, so they no longer need to be checked for re-exporting.

[cherylEnkidu]

Tests involving the Firestore emulator are still failing, but aside from that, this PR improves overall code quality. I suggest merging it and opening a separate PR to investigate the emulator failures.
Since I contributed to this PR, I’d like to ask @wu-hui to be the reviewer.