Use 1 of the same type as the stored value when shifting left. This
prevents undefined behavior caused by shifting an int more than 31 bits.

Previously iterator incrementation could either hang or prematurely
report it had reached the end of the bitset.

Group numbers and their bitcounts together in pairs, which allows for
range-based loop iteration.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Using `EXPECT_EQ` with boolean literals can cause a warning in certain
versions of GCC. See https://github.com/google/googletest/issues/322

Fixes warnings:

```
../Source/UnitTests/Common/BitSetTest.cpp: In member function 'virtual void BitSet_Basics_Test::TestBody()':
../Source/UnitTests/Common/BitSetTest.cpp:15:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/BitSetTest.cpp: In member function 'virtual void BitSet_BitGetSet_Test::TestBody()':
../Source/UnitTests/Common/BitSetTest.cpp:27:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp: In member function 'virtual void StringUtil_StringBeginsWith_Test::TestBody()':
../Source/UnitTests/Common/StringUtilTest.cpp:23:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp:25:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp:26:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp:27:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp: In member function 'virtual void StringUtil_StringEndsWith_Test::TestBody()':
../Source/UnitTests/Common/StringUtilTest.cpp:35:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp:37:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp:38:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
../Source/UnitTests/Common/StringUtilTest.cpp:39:165: error: converting 'false' to pointer type for argument 1 of 'char testing::internal::IsNullLiteralHelper(testing::internal::Secret*)' [-Werror=conversion-null]
c
```