[clang-tools-extra] [clang-doc] add Markdown parser (PR #155887)

[Paul Kirth via cfe-commits]

================
@@ -0,0 +1,99 @@
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_DOC_MD_PARSER_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_DOC_MD_PARSER_H
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/StringSaver.h"
+#include <list>
+
+using namespace llvm;
+
+namespace clang {
+namespace doc {
+using llvm::SmallString;
+enum class MDState { Emphasis, Strong, None };
+
+enum class MDType {
+  Paragraph,
+  Emphasis,
+  Strong,
+  Text,
+  Softbreak,
+};
+
+enum class MDTokenType { LeftDelimiterRun, RightDelimiterRun, Text };
+
+struct Node {
+  SmallVector<Node*> Children;
+  MDType Type;
+  Node *Parent;
+  std::string Content;
+};
+
+struct DelimiterContext {
+  bool RightFlanking;
+  bool LeftFlanking;
+  bool CanOpen;
+  bool CanClose;
+  char DelimChar;
+  // Since Content is a StringRef, we separately track the length so that we can
+  // decrement when necessary without modifying the string.
+  size_t Length;
+};
----------------
ilovepi wrote:

```suggestion
struct DelimiterContext {
  // Since Content is a StringRef, we separately track the length so that we can
  // decrement when necessary without modifying the string.
  size_t Length;
  char DelimChar;
  bool RightFlanking;
  bool LeftFlanking;
  bool CanOpen;
  bool CanClose;
};
```
lets order the fields by size, so we avoid padding. 

https://github.com/llvm/llvm-project/pull/155887