[clang] [TySan] Add initial documentation for Type Sanitizer (PR #123595)

[Erich Keane via cfe-commits]

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+================
+TypeSanitizer
+================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+TypeSanitizer is a detector for strict type aliasing violations. It consists of a compiler
+instrumentation module and a run-time library. The tool detects violations where you access 
+memory under a different type than the dynamic type of the object.
+
+The violations TypeSanitizer catches may cause the compiler to emit incorrect code.
+
+As TypeSanitizer is still experimental, it can currently have a large impact on runtime speed, 
+memory use, and code size.
+
+How to build
+============
+
+Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_ and enable
+the ``compiler-rt`` runtime. An example CMake configuration that will allow
+for the use/testing of TypeSanitizer:
+
+.. code-block:: console
+
+   $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="compiler-rt" <path to source>/llvm
+
+Usage
+=====
+
+Compile and link your program with ``-fsanitize=type`` flag.  The
+TypeSanitizer run-time library should be linked to the final executable, so
+make sure to use ``clang`` (not ``ld``) for the final link step. To
+get a reasonable performance add ``-O1`` or higher.
+TypeSanitizer by default doesn't print the full stack trace on error messages. Use ``TYSAN_OPTIONS=print_stacktrace=1`` 
----------------
erichkeane wrote:

```suggestion
TypeSanitizer by default doesn't print the full stack trace in error messages. Use ``TYSAN_OPTIONS=print_stacktrace=1`` 
```

https://github.com/llvm/llvm-project/pull/123595