[CIRGen] Task 1: Support for builtin __atomic_thread_fence

Part of #1274

Implements atomic thread fence synchronization primitive
corresponding to `atomic.thread_fence` CIR.

Author: Rajveer100

clang/include/clang/CIR/Dialect/IR/CIROps.td

@@ -5403,6 +5403,50 @@ def AtomicCmpXchg : CIR_Op<"atomic.cmp_xchg",
   let hasVerifier = 0;
 }
 
+def MemScopeEnum : EnumAttr<"Memory scope enumeration"> {
+  let underlying_type = "uint32_t";
+  let enumerants = [
+    Enumerant<"single_thread", "Applies to the calling thread only", 0>
+    Enumerant<"system", "Spans the entire system", 1>
+  ];
+}
+
+def AtomicOrderingEnum : EnumAttr<"Atomic memory ordering enumeration"> {
+  let underlying_type = "uint32_t";
+  let enumerants = [
+    Enumerant<"relaxed", "No ordering constraints", 0>,
+    Enumerant<"acquire", "All subsequent memory operations occur after this fence", 1>,
+    Enumerant<"release", "All prior memory operations occur before this fence", 2>,
+    Enumerant<"acq_rel", "Combines acquire and release semantics", 3>,
+    Enumerant<"seq_cst", "Enforces sequential consistency", 4>
+  ];
+};
+
+def AtomicFenceOp : CIR_Op<"atomic.thread_fence"> {
+  let summary = "Atomic thread fence";
+  let description = [{
+    C/C++ Atomic thread fence synchronization primitive. Implements the builtin
+    `__atomic_thread_fence` which enforces memory ordering constraints across
+    threads within the specified synchronization scope.
+
+    Example:
+
+
+  }];
+  let results = (outs);
+  let arguments = (ins Attr<EnumAttr<["single_thread", "system"]>,
+                            "synchronization scope":$sync_scope,
+                       Attr<EnumAttr<["relaxed", "acquire", "release", "acq_rel", "seq_cst"]>,
+                            "atomic memory ordering":$ordering);
+
+  let assemblyFormat = [{
+    `(` `sync_scope` `=` $sync_scope `,`
+        `ordering` `=` $ordering `)` attr-dict
+  }];
+
+  let hasVerifier = 1;
+}
+
 def SignBitOp : CIR_Op<"signbit", [Pure]> {
   let summary = "Checks the sign of a floating-point number";
   let description = [{