Inconsistent file header licensing

It has been observed that we have recently introduced a bunch of new files with varying license headers:

| License Type | File |
|----------------|------------------------------------------------------------------------------------------------------------------------------------------------|
|`Apache-2.0` | [`gtests/HexFloat.cpp`](https://github.com/KhronosGroup/glslang/blob/master/gtests/HexFloat.cpp) |
|`Apache-2.0` | [`ndk_test/test.cpp`](https://github.com/KhronosGroup/glslang/blob/master/ndk_test/test.cpp) |
|`Apache-2.0` | [`SPIRV/bitutils.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/bitutils.h) |
|`Apache-2.0` | [`SPIRV/hex_float.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/hex_float.h) |
|`Apache-2.0` | [`update_glslang_sources.py`](https://github.com/KhronosGroup/glslang/blob/master/update_glslang_sources.py) |
|`BSD-2-Clause` | [`glslang/CInterface/glslang_c_interface.cpp`](https://github.com/KhronosGroup/glslang/blob/master/glslang/CInterface/glslang_c_interface.cpp) |
|`BSD-2-Clause` | [`glslang/Include/glslang_c_interface.h`](https://github.com/KhronosGroup/glslang/blob/master/glslang/Include/glslang_c_interface.h) |
|`BSD-2-Clause` | [`glslang/Include/glslang_c_shader_types.h`](https://github.com/KhronosGroup/glslang/blob/master/glslang/Include/glslang_c_shader_types.h) |
|`BSD-2-Clause` | [`SPIRV/CInterface/spirv_c_interface.cpp`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/CInterface/spirv_c_interface.cpp) |
|`BSD-2-Clause` | [`StandAlone/resource_limits_c.cpp`](https://github.com/KhronosGroup/glslang/blob/master/StandAlone/resource_limits_c.cpp) |
|`BSD-2-Clause` | [`StandAlone/resource_limits_c.h`](https://github.com/KhronosGroup/glslang/blob/master/StandAlone/resource_limits_c.h) |
|`MIT` | [`SPIRV/GLSL.ext.AMD.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/GLSL.ext.AMD.h) |
|`MIT` | [`SPIRV/GLSL.ext.EXT.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/GLSL.ext.EXT.h) |
|`MIT` | [`SPIRV/GLSL.ext.KHR.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/GLSL.ext.KHR.h) |
|`MIT` | [`SPIRV/GLSL.ext.NV.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/GLSL.ext.NV.h) |
|`MIT` | [`SPIRV/GLSL.std.450.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/GLSL.std.450.h) |
|`MIT` | [`SPIRV/NonSemanticDebugPrintf.h`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/NonSemanticDebugPrintf.h) |
|`MIT` | [`SPIRV/spirv.hpp`](https://github.com/KhronosGroup/glslang/blob/master/SPIRV/spirv.hpp) |
|`MIT` | [`glslang/MachineIndependent/gl_types.h`](https://github.com/KhronosGroup/glslang/blob/master/glslang/MachineIndependent/gl_types.h) |

Note the bison generated files [`glslang/MachineIndependent/glslang_tab.cpp`](https://github.com/KhronosGroup/glslang/blob/master/glslang/MachineIndependent/glslang_tab.cpp) and [`glslang/MachineIndependent/glslang_tab.cpp.h`](https://github.com/KhronosGroup/glslang/blob/master/glslang/MachineIndependent/glslang_tab.cpp.h) use yet another license type ([a GPL-3.0 license with a special exemption](https://github.com/KhronosGroup/glslang/blob/b112fac0033c92810ce6c477b2fe36011d184ee0/glslang/MachineIndependent/glslang_tab.cpp.h#L20-L31)). These have always been this way, and this issue does not relate to these files.

Despite the [Khronos CLA](https://cla-assistant.io/KhronosGroup/glslang) suggesting that the project should be using the MIT License, [the standard license used by glslang is BSD-3-clause](https://github.com/KhronosGroup/glslang/blob/b112fac0033c92810ce6c477b2fe36011d184ee0/LICENSE.txt#L26-L60).

We need to either (and/or):
1. Add the new license types to [the `LICENSE.txt` file](https://github.com/KhronosGroup/glslang/blob/master/LICENSE.txt) (done: #2306)
2. Fix up the license headers of the files listed above to be BSD-3-clause.

Start by reviewing LICENSE.txt and the listed source and header files, excluding the two bison-generated files explicitly out of scope. Decide which listed headers need BSD-3-Clause notices, then update the headers consistently and verify that the license types referenced by the files are documented in LICENSE.txt.

Written by the indexing model from the issue text.