[clang-doc] add a JSON generator (#142483)

Adds a JSON generator backend to emit mapped information as JSON. This will enable a better testing format for upcoming changes. It can also potentially serve to feed our other backend generators in the future, like Mustache which already serializes information to JSON before emitting as HTML.

This patch contains functionality to emit classes and provides most of the basis of the generator.

Author: evelez7

clang-tools-extra/clang-doc/CMakeLists.txt

@@ -17,6 +17,7 @@ add_clang_library(clangDoc STATIC
   Serialize.cpp
   YAMLGenerator.cpp
   HTMLMustacheGenerator.cpp
+  JSONGenerator.cpp
 
   DEPENDS
   omp_gen

clang-tools-extra/clang-doc/Generators.cpp

@@ -105,5 +105,7 @@ static int LLVM_ATTRIBUTE_UNUSED HTMLGeneratorAnchorDest =
     HTMLGeneratorAnchorSource;
 static int LLVM_ATTRIBUTE_UNUSED MHTMLGeneratorAnchorDest =
     MHTMLGeneratorAnchorSource;
+static int LLVM_ATTRIBUTE_UNUSED JSONGeneratorAnchorDest =
+    JSONGeneratorAnchorSource;
 } // namespace doc
 } // namespace clang

clang-tools-extra/clang-doc/Generators.h

@@ -58,6 +58,7 @@ extern volatile int YAMLGeneratorAnchorSource;
 extern volatile int MDGeneratorAnchorSource;
 extern volatile int HTMLGeneratorAnchorSource;
 extern volatile int MHTMLGeneratorAnchorSource;
+extern volatile int JSONGeneratorAnchorSource;
 
 } // namespace doc
 } // namespace clang