[CIR] Upstream missing support for scalar expressions

Overview

Upstream code from the incubator project to replace errorNYI calls in the
CIR codegen implementation related to scalar expressions:

• Nontemporal stores: threading the isNontemporal flag through to
  builder.createStore()
• Void type loads: converting void element type to UInt8 via
  withElementType before loading
• Scalar constant references: loading through a reference returned by
  ConstantEmission::getReferenceLValue()
• Scalar materialize temporary: calling emitLoadOfLValue(E) to load
  through a materialized temporary's lvalue
• Pseudo object expressions: delegating to
  CGF.emitPseudoObjectRValue(E).getScalarVal() for pseudo-object rvalue
  emission

Upstream NYI locations:

• CIRGenExpr.cpp — emitStoreOfScalar: nontemporal store flag
• CIRGenExpr.cpp — emitLoadOfScalar: void element type
• CIRGenExpr.cpp — emitScalarConstant: reference constant emission
• CIRGenExprScalar.cpp — VisitMaterializeTemporaryExpr: scalar temp load
• CIRGenExprScalar.cpp — VisitPseudoObjectExpr: pseudo object rvalue

Suggested minimal test case

// Nontemporal store
void test_nontemporal(int *p, int v) {
  __builtin_nontemporal_store(v, p);
}

// Scalar constant reference
const int &get_ref();
int test_const_ref() {
  static constexpr int val = 42;
  const int &r = val;
  return r;
}

// Materialize temporary (scalar)
int test_materialize() {
  return (const int &)42;
}

Existing incubator tests

• clang/test/CIR/CodeGen/builtin-nontemporal.cpp
• clang/test/CIR/CodeGen/temporary-materialization.cpp
• clang/test/CIR/CodeGen/materialize-temporary.cpp