[clang-tools-extra] [clang-doc] Introduce Serializer class (PR #184873)

[Carlos Alberto Enciso via cfe-commits]

CarlosAlbertoEnciso wrote:

@ilovepi The patch is causing some compilation errors on Linux and Windows
Linux
```
.../clang-tools-extra/clang-doc/benchmarks/ClangDocBenchmark.cpp: In function ‘void clang::doc::BM_EmitInfoFunction(benchmark::State&)’:
.../clang-tools-extra/clang-doc/benchmarks/ClangDocBenchmark.cpp:63:16: error: ‘ClangDocSerializer’ is not a member of ‘clang::doc::serialize’
   63 |     serialize::ClangDocSerializer Serializer;
      |                ^~~~~~~~~~~~~~~~~~
.../clang-tools-extra/clang-doc/benchmarks/ClangDocBenchmark.cpp:64:19: error: ‘Serializer’ was not declared in this scope; did you mean ‘clang::doc::serialize::Serializer’?
   64 |     auto Result = Serializer.emitInfo(Func, FC, Loc, /*PublicOnly=*/false);
      |                   ^~~~~~~~~~
      |                   clang::doc::serialize::Serializer
In file included from .../clang-tools-extra/clang-doc/benchmarks/ClangDocBenchmark.cpp:20:
.../clang-tools-extra/clang-doc/benchmarks/../Serialize.h:36:7: note: ‘clang::doc::serialize::Serializer’ declared here
   36 | class Serializer {
      |       ^~~~~~~~~~
```
Windows
```
1>ClangDocBenchmark.cpp
1>...\clang-tools-extra\clang-doc\benchmarks\ClangDocBenchmark.cpp(63,16): error C2039: 'ClangDocSerializer': is not a member of 'clang::doc::serialize'
1>...\clang-tools-extra\clang-doc\benchmarks\..\Serialize.h(25): message : see declaration of 'clang::doc::serialize'
1>...\clang-tools-extra\clang-doc\benchmarks\ClangDocBenchmark.cpp(63,35): error C2065: 'ClangDocSerializer': undeclared identifier
1>...\clang-tools-extra\clang-doc\benchmarks\ClangDocBenchmark.cpp(63,35): error C2146: syntax error: missing ';' before identifier 'Serializer'
1>...\clang-tools-extra\clang-doc\benchmarks\ClangDocBenchmark.cpp(63,35): error C2065: 'Serializer': undeclared identifier
1>...\clang-tools-extra\clang-doc\benchmarks\ClangDocBenchmark.cpp(64,19): error C2065: 'Serializer': undeclared identifier
1>...\clang-tools-extra\clang-doc\benchmarks\ClangDocBenchmark.cpp(65,36): error C3536: 'Result': cannot be used before it is initialized
```

https://github.com/llvm/llvm-project/pull/184873