[clang] [clang][CIR][doc] Add auto-generated ClangIR documentation (PR #190354)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,23 @@
+=======
+ClangIR
+=======
+
+.. warning::
+    The project of upstreaming ClangIR support from the incubator repository is
+    still in progress, and ClangIR is not included in a default clang build. The
+    documentation may be incomplete and out-of-date.
+
+ClangIR is a high-level representation in Clang that reflects aspects of the
+C/C++ languages and their extensions. It is implemented using MLIR and occupies
+a position between Clang's AST and LLVM IR.
+
+ClangIR Design Documents
+========================
+
+.. toctree::
+    :numbered:
+    :maxdepth: 1
+
+    ABILowering
+    CleanupAndEHDesign
----------------
AaronBallman wrote:

Not as part of this PR, but these should be converted to be .rst files instead of .md files.

https://github.com/llvm/llvm-project/pull/190354