[clang] [CIR] Upstream minimal support for structure types (PR #135105)

[Andy Kaylor via cfe-commits]

================
@@ -400,13 +400,126 @@ def VoidPtr : Type<
       "cir::VoidType::get($_builder.getContext()))"> {
 }
 
+//===----------------------------------------------------------------------===//
+// StructType
+//
+// The base type for all RecordDecls.
+//===----------------------------------------------------------------------===//
+
+def CIR_StructType : CIR_Type<"Struct", "struct",
+    [
+      DeclareTypeInterfaceMethods<DataLayoutTypeInterface>,
+      MutableType,
+    ]> {
+  let summary = "CIR struct type";
+  let description = [{
+    Each unique clang::RecordDecl is mapped to a `cir.struct` and any object in
+    C/C++ that has a struct type will have a `cir.struct` in CIR.
+
+    There are three possible formats for this type:
+
+     - Identified and complete structs: unique name and a known body.
+     - Identified and incomplete structs: unique name and unknown body.
+     - Anonymous structs: no name and a known body.
+
+    Identified structs are uniqued by their name, and anonymous structs are
+    uniqued by their body. This means that two anonymous structs with the same
+    body will be the same type, and two identified structs with the same name
+    will be the same type. Attempting to build a struct with an existing name,
+    but a different body will result in an error.
+
+    A few examples:
+
+    ```mlir
+        !complete = !cir.struct<struct "complete" {!cir.int<u, 8>}>
+        !incomplete = !cir.struct<struct "incomplete" incomplete>
+        !anonymous = !cir.struct<struct {!cir.int<u, 8>}>
+    ```
+
+    Incomplete structs are mutable, meaning they can be later completed with a
+    body automatically updating in place every type in the code that uses the
+    incomplete struct. Mutability allows for recursive types to be represented,
+    meaning the struct can have members that refer to itself. This is useful for
+    representing recursive records and is implemented through a special syntax.
+    In the example below, the `Node` struct has a member that is a pointer to a
+    `Node` struct:
+
+    ```mlir
+        !struct = !cir.struct<struct "Node" {!cir.ptr<!cir.struct<struct
----------------
andykaylor wrote:

@bcardosolopes Any objections to renaming `cir.struct` to `cir.record`?

https://github.com/llvm/llvm-project/pull/135105