Add SPIR-V const-folding regression test for int64_t and uint64_t

perform tests on new `tools\clang\test\CodeGenSPIRV\spv.constant-folding.template.int64.hlsl` with `llvm-lit.py`, before the fix and after

Author: AnastaZIuk

tools/clang/test/CodeGenSPIRV/spv.constant-folding.template.int64.hlsl

@@ -0,0 +1,60 @@
+// RUN: %dxc -O3 -T lib_6_8 -HV 2021 -spirv -fcgl -fspv-target-env=universal1.5 %s | FileCheck %s
+
+template <typename S> struct Trait;
+
+template <> struct Trait<int64_t> {
+  using type = int64_t;
+  static const type value = -42;
+};
+
+Trait<int64_t>::type get_s64_value() {
+  return Trait<int64_t>::value;
+}
+
+int64_t cvt_s64(int64_t x) { return int64_t(x); }
+
+template <> struct Trait<uint64_t> {
+  using type = uint64_t;
+  static const type value = 1337ull;
+};
+
+Trait<uint64_t>::type get_u64_value() {
+  return Trait<uint64_t>::value;
+}
+
+uint64_t cvt_u64(uint64_t x) { return uint64_t(x); }
+
+// CHECK-LABEL: %testcase_s64 = OpFunction %long
+export int64_t testcase_s64(int x) {
+  if (x == 2) {
+    // CHECK: OpReturnValue %long_n42
+    return int64_t(Trait<int64_t>::value);
+  }
+  else if (x == 4) {
+    // CHECK: OpStore %param_var_x %long_n42
+    // CHECK-NEXT: OpFunctionCall %long %cvt_s64 %param_var_x
+    return cvt_s64(Trait<int64_t>::value);
+  } else if (x == 8) {
+    return get_s64_value();
+  }
+  return 0ll;
+}
+
+// CHECK-LABEL: %testcase_u64 = OpFunction %ulong
+export uint64_t testcase_u64(int x) {
+  if (x == 2) {
+    // CHECK: OpReturnValue %ulong_1337
+    return uint64_t(Trait<uint64_t>::value);
+  }
+  else if (x == 4) {
+    // CHECK: OpStore %param_var_x_0 %ulong_1337
+    // CHECK-NEXT: OpFunctionCall %ulong %cvt_u64 %param_var_x_0
+    return cvt_u64(Trait<uint64_t>::value);
+  } else if (x == 8) {
+    return get_u64_value();
+  }
+  return 0ull;
+}
+
+// CHECK-LABEL: %get_s64_value = OpFunction %long
+// CHECK-LABEL: %get_u64_value = OpFunction %ulong