[clang] [CIR] Add support for function linkage and visibility (PR #145600)

[Andy Kaylor via cfe-commits]

================
@@ -1737,25 +1737,51 @@ def GetMemberOp : CIR_Op<"get_member"> {
 
 def FuncOp : CIR_Op<"func", [
   AutomaticAllocationScope, CallableOpInterface, FunctionOpInterface,
+  DeclareOpInterfaceMethods<CIRGlobalValueInterface>,
   IsolatedFromAbove
 ]> {
   let summary = "Declare or define a function";
   let description = [{
     The `cir.func` operation defines a function, similar to the `mlir::FuncOp`
     built-in.
+
+    The function linkage information is specified by `linkage`, as defined by
+    `GlobalLinkageKind` attribute.
+
+    Example:
+
+    ```mlir
+    // External function definitions.
+    cir.func @abort()
+
+    // A function with internal linkage.
+    cir.func internal @count(%x: i64) -> (i64)
+      return %x : i64
+
+    // Linkage information
+    cir.func linkonce_odr @some_method(...)
+    ```
   }];
 
   let arguments = (ins SymbolNameAttr:$sym_name,
+                       CIR_VisibilityAttr:$global_visibility,
                        TypeAttrOf<CIR_FuncType>:$function_type,
+                       UnitAttr:$dso_local,
----------------
andykaylor wrote:

There's a 1-to-1 correspondance between this and the `dso_local` representation in LLVM IR, and we are able to model the computation of this flag after the same computation in classic codegen. I don't think there's benefit in trying to generalize this as an intermediate state when we'll need to recreate it when lowering to LLVM IR anyway.

https://github.com/llvm/llvm-project/pull/145600