Tint changes during Origin Trial

Changes for M95

New Features

• The size of an array can now be defined using a non-overridable module-scope constant
• The num_workgroups builtin is now supported.

Fixes

• Hex floats: issue an error when the magnitude is non-zero, and the exponent would cause overflow. https://crbug.com/tint/1150 https://crbug.com/tint/1166
• Reject identifiers beginning with an underscore. https://crbug.com/tint/1179