tools/clang/json: Escape strings properly

When preparing a codesearch index, I encountered errors which I narrowed
down to lines like the following in the json output of codesearch:

  "type": "void (void  __attribute__((btf_type_tag("user")))*, const void *, size_t, size_t)",

After this change, the line gets formatted like this:

  "type": "void (void  __attribute__((btf_type_tag(\"user\")))*, const void *, size_t, size_t)",

This fixes the errors I encountered

Author: FlorentRevest

pkg/codesearch/testdata/query-def-source-quote-type

@@ -0,0 +1,7 @@
+def-source source0.c function_with_quotes_in_type yes
+
+function function_with_quotes_in_type is defined in source0.c:
+
+  29:	void function_with_quotes_in_type(void __attribute__((btf_type_tag("user")))*)
+  30:	{
+  31:	}

pkg/codesearch/testdata/query-file-index-source

@@ -4,5 +4,6 @@ file source0.c defines the following entities:
 
 function close
 function function_with_comment_in_header
+function function_with_quotes_in_type
 function open
 struct struct_in_c_file

pkg/codesearch/testdata/source0.c

@@ -25,3 +25,7 @@ void function_with_comment_in_header()
 {
 	same_name_in_several_files();
 }
+
+void function_with_quotes_in_type(void __attribute__((btf_type_tag("user")))*)
+{
+}

pkg/codesearch/testdata/source0.c.json

@@ -52,6 +52,17 @@
 				}
 			]
 		},
+		{
+			"kind": "function",
+			"name": "function_with_quotes_in_type",
+			"type": "void (void  __attribute__((btf_type_tag(\"user\")))*)",
+			"body": {
+				"file": "source0.c",
+				"start_line": 29,
+				"end_line": 31
+			},
+			"comment": {}
+		},
 		{
 			"kind": "function",
 			"name": "open",

tools/clang/json.h

@@ -52,11 +52,33 @@ class JSONPrinter {
   Scope Top;
 };
 
+inline void print(JSONPrinter& Printer, const char* V) {
+  printf("\"");
+  if (V) {
+    for (; *V; ++V) {
+      switch (*V) {
+      case '"': printf("\\\""); break;
+      case '\\': printf("\\\\"); break;
+      case '\b': printf("\\b"); break;
+      case '\f': printf("\\f"); break;
+      case '\n': printf("\\n"); break;
+      case '\r': printf("\\r"); break;
+      case '\t': printf("\\t"); break;
+      default:
+        if ((unsigned char)*V < 0x20)
+          printf("\\u%04x", (unsigned char)*V);
+        else
+          printf("%c", *V);
+      }
+    }
+  }
+  printf("\"");
+}
+
 inline void print(JSONPrinter& Printer, int V) { printf("%d", V); }
 inline void print(JSONPrinter& Printer, unsigned V) { printf("%u", V); }
 inline void print(JSONPrinter& Printer, int64_t V) { printf("%ld", V); }
 inline void print(JSONPrinter& Printer, bool V) { printf("%s", V ? "true" : "false"); }
-inline void print(JSONPrinter& Printer, const char* V) { printf("\"%s\"", V ? V : ""); }
 inline void print(JSONPrinter& Printer, const std::string& V) { print(Printer, V.c_str()); }
 
 template <typename E> void print(JSONPrinter& Printer, const std::unique_ptr<E>& V) {