[MLIR][Target/Cpp] Fix variable naming conflict for function declarations (#147927)

This is a fix for #136102. It
missed scoping for `DeclareFuncOps`.
In scenarios with multiple function declarations, the `valueMapper`
wasn't updated and later uses of values in other functions still used
the assigned names in prior functions.

This is visible in the reproducer here
iree-org/iree#21303: Although the counter for
variable enumeration was reset, as it is visible for the local vars, the
function arguments were mapped to old names. Due to this mapping, the
counter was never increased, and the local variables conflicted with the
arguments.

This fix adds proper scoping for declarations and a test-case to cover
the scenario with multiple `DeclareFuncOps`.

Author: ndegener-amd

mlir/lib/Target/Cpp/TranslateToCpp.cpp

@@ -1258,6 +1258,7 @@ static LogicalResult printOperation(CppEmitter &emitter,
                                     DeclareFuncOp declareFuncOp) {
   raw_indented_ostream &os = emitter.ostream();
 
+  CppEmitter::FunctionScope scope(emitter);
   auto functionOp = SymbolTable::lookupNearestSymbolFrom<emitc::FuncOp>(
       declareFuncOp, declareFuncOp.getSymNameAttr());
 

mlir/test/Target/Cpp/declare_func.mlir

@@ -24,3 +24,20 @@ emitc.declare_func @array_arg
 emitc.func @array_arg(%arg0: !emitc.array<3xi32>) {
     emitc.return
 }
+
+// CHECK: int32_t foo1(int32_t [[V1:[^ ]*]]);
+emitc.declare_func @foo1
+// CHECK: int32_t foo2(int32_t [[V1]]);
+emitc.declare_func @foo2
+// CHECK: int32_t foo1(int32_t [[V1]]) {
+emitc.func @foo1(%arg0: i32) -> i32 {
+    // CHECK-NOT: int32_t [[V1]] = 0;
+    %0 = "emitc.constant"() <{value = 0 : i32}> : () -> i32
+    // CHECK: return [[V1]];
+    emitc.return %arg0 : i32
+}
+// CHECK: int32_t foo2(int32_t [[V1]]) {
+emitc.func @foo2(%arg0: i32) -> i32 {
+    // CHECK: return [[V1]];
+    emitc.return %arg0 : i32
+}