spirv: Generate missing SampleMaskOverrideCoverageNV capability op

Explicitly remove RTTI in the top-level build config

The SampleMaskOverrideCoverageNV capability should be declared when
gl_SampleMask is used.

Fixes #1831

Add support for SPV_KHR_shader_clock

Avoid generating 8/16-bit constants when 8/16-bit arithmetic extensions aren't enabled

Update spirv tools

Change implementation of gl_SIMDGroupSizeAMD

update spirv-headers to pick up demote_to_helper_invocation

Implement GL_EXT_demote_to_helper_invocation

Remove extraneous semicolons

Remove __DATE__ and __TIME__ usage

These macros result in a non-deterministic build.  In chromium, these
macros are specifically set to empty, resulting in a compile error.

They cause a warning (-Wextra-semi) that can lead to a compile error
(-Werror).

Handle all basic scalar types in promoteConstantUnion

Add a bunch of missing cases to promoteConstantUnion. Use macros to expand all the cases more simply.

BUILD.gn: Fix file for use with Fuchsia platform build.

In order to upgrade the version of glslang used by the
Fuchsia platform source tree, BUILD.gn needs to be
slightly modified to care about the case where it is
not used with the Chromium //build configuration:

- Remove a new compiler warning to ensure proper
  compilation with -Werror (which is the default).

- Add a build target for spirv-remap, which is used
  by Fuchsia at build time to optimize the precompiled
  shaders of some of its graphics libraries.

AST/SPV: Fix #930: translate uvec4 <-> uint64 for SubgroupGeMask et. al.

On reading built-in variables SubgroupEqMask, SubgroupGeMask, SubgroupGtMask,
SubgroupLeMask, and SubgroupLtMask, the AST expects 64-bit ints, while SPIR-V
is defined as vectors of 32-bit ints.

The declaration type has to be translated in the opposite direction.

SPV: Add a switch for favoring non-NaN operands in min, max, and clamp.

Add Float16/Int8/Int16 capabilities for private variables and function parameters

Fix gl_SubGroupSizeARB builtin treatment