[clang] [CIR] Implement inline builtin functions (PR #163911)

[Henrich Lauko via cfe-commits]

================
@@ -551,6 +551,50 @@ cir::FuncOp CIRGenFunction::generateCode(clang::GlobalDecl gd, cir::FuncOp fn,
   const auto funcDecl = cast<FunctionDecl>(gd.getDecl());
   curGD = gd;
 
+  if (funcDecl->isInlineBuiltinDeclaration()) {
+    // When generating code for a builtin with an inline declaration, use a
+    // mangled name to hold the actual body, while keeping an external
+    // declaration in case the function pointer is referenced somewhere.
+    std::string fdInlineName = fn.getName().str() + ".inline";
+    cir::FuncOp clone =
+        mlir::cast_or_null<cir::FuncOp>(cgm.getGlobalValue(fdInlineName));
+    if (!clone) {
+      mlir::OpBuilder::InsertionGuard guard(builder);
+      builder.setInsertionPoint(fn);
+      clone = builder.create<cir::FuncOp>(fn.getLoc(), fdInlineName,
+                                          fn.getFunctionType());
+      clone.setLinkageAttr(cir::GlobalLinkageKindAttr::get(
+          &cgm.getMLIRContext(), cir::GlobalLinkageKind::InternalLinkage));
+      clone.setSymVisibility("private");
+      clone.setInlineKindAttr(cir::InlineAttr::get(
+          &cgm.getMLIRContext(), cir::InlineKind::AlwaysInline));
+    }
+    fn.setLinkageAttr(cir::GlobalLinkageKindAttr::get(
+        &cgm.getMLIRContext(), cir::GlobalLinkageKind::ExternalLinkage));
+    fn.setSymVisibility("private");
----------------
xlauko wrote:

I believe you should be able to use `setLinkage` and `setInlineKind` without explicit attribute creation:

```suggestion
      clone.setLinkage(cir::GlobalLinkageKind::InternalLinkage);
      clone.setSymVisibility("private");
      clone.setInlineKind(cir::InlineKind::AlwaysInline);
    }
    fn.setLinkage(cir::GlobalLinkageKind::ExternalLinkage);
    fn.setSymVisibility("private");
```

https://github.com/llvm/llvm-project/pull/163911