Add BUILD.gn configuration.

This CL adds the necessary configuration to build glslang inside a
Chromium checkout. Two build warnings were fixed in the process to
make things compile.

Update known-good to tools/headers for the Nvidia Turing extensions.

Add support for multiple Nvidia extensions

Add GL_EXT_shader_atomic_int64

Fix compiler warning emitted from GCC8

the following warning gets emitted:

```
In file included from ./Vulkan/glslang/SPIRV/hex_float.h:39,
                 from ./Vulkan/glslang/SPIRV/SpvBuilder.cpp:49:
./Vulkan/glslang/SPIRV/bitutils.h: In instantiation of ‘Dest spvutils::BitwiseCast(Src) [with Dest = spvutils::Float16; Src = short unsigned int]’:
./Vulkan/glslang/SPIRV/hex_float.h:138:47:   required from ‘T spvutils::FloatProxy<T>::getAsFloat() const [with T = spvutils::Float16]’
./Vulkan/glslang/SPIRV/hex_float.h:821:52:   required from here
./Vulkan/glslang/SPIRV/bitutils.h:29:14: warning: ‘void* memcpy(void*, const void*, size_t)’ writing to an object of non-trivially copyable type ‘class spvutils::Float16’; use copy-assignment or copy-initialization instead [-Wclass-memaccess]
   std::memcpy(&dest, &source, sizeof(dest));
   ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./Vulkan/glslang/SPIRV/SpvBuilder.cpp:49:
./Vulkan/glslang/SPIRV/hex_float.h:43:7: note: ‘class spvutils::Float16’ declared here
 class Float16 {
       ^~~~~~~
In file included from ./Vulkan/glslang/SPIRV/hex_float.h:39,
                 from ./Vulkan/glslang/SPIRV/SpvBuilder.cpp:49:
./Vulkan/glslang/SPIRV/bitutils.h: In instantiation of ‘Dest spvutils::BitwiseCast(Src) [with Dest = spvutils::FloatProxy<spvutils::Float16>; Src = short unsigned int]’:
./Vulkan/glslang/SPIRV/hex_float.h:431:28:   required from ‘void spvutils::HexFloat<T, Traits>::setFromSignUnbiasedExponentAndNormalizedSignificand(bool, spvutils::HexFloat<T, Traits>::int_type, spvutils::HexFloat<T, Traits>::uint_type, bool) [with T = spvutils::FloatProxy<spvutils::Float16>; Traits = spvutils::HexFloatTraits<spvutils::FloatProxy<spvutils::Float16> >; spvutils::HexFloat<T, Traits>::int_type = short int; spvutils::HexFloat<T, Traits>::uint_type = short unsigned int]’
./Vulkan/glslang/SPIRV/hex_float.h:633:5:   required from ‘void spvutils::HexFloat<T, Traits>::castTo(other_T&, spvutils::round_direction) [with other_T = spvutils::HexFloat<spvutils::FloatProxy<spvutils::Float16>, spvutils::HexFloatTraits<spvutils::FloatProxy<spvutils::Float16> > >; T = spvutils::FloatProxy<float>; Traits = spvutils::HexFloatTraits<spvutils::FloatProxy<float> >]’
./Vulkan/glslang/SPIRV/hex_float.h:817:39:   required from here
./Vulkan/glslang/SPIRV/bitutils.h:29:14: warning: ‘void* memcpy(void*, const void*, size_t)’ copying an object of non-trivial type ‘class spvutils::FloatProxy<spvutils::Float16>’ from an array of ‘short unsigned int’ [-Wclass-memaccess]
   std::memcpy(&dest, &source, sizeof(dest));
   ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./Vulkan/glslang/SPIRV/SpvBuilder.cpp:49:
./Vulkan/glslang/SPIRV/hex_float.h:115:7: note: ‘class spvutils::FloatProxy<spvutils::Float16>’ declared here
 class FloatProxy {
       ^~~~~~~~~~
```

Put quote marks around strings, due to a change in how tokenization works.

Remove execute permissions

tooling: Fix update sources script for Python 3

Fixed a typo leading to compilation error.

Fix build.

Also use the glslang namespace qualifier consistently and validate after
legalization, not before. (But most tests don't legalize.)

SPV: Add SPIRV-Tools validator.