Make Clang 6 the basic build test configuration

[BrettDong]

Summary

None

Purpose of change

The Clang 6, Ubuntu, Curses configuration compiles and links much faster than GCC 9 LTO, and tests also finish quicker than the GCC 9 LTO workflow.

Sample 1: https://github.com/CleverRaven/Cataclysm-DDA/actions/runs/1739045814

Test cases	GCC 9 LTO	Clang 6
(crafting_skill_gain)	34.5541 seconds	19.933 seconds
(~[slow] ~[.])	409.762 seconds	336.785 seconds
([slow] ~crafting_skill_gain)	707.586 seconds	458.861 seconds

Sample 2: https://github.com/CleverRaven/Cataclysm-DDA/actions/runs/1786607218

Test cases	GCC 9 LTO	Clang 6
(crafting_skill_gain)	26.0063 seconds	21.6418 seconds
(~[slow] ~[.])	151.917 seconds	126.575 seconds
([slow] ~crafting_skill_gain)	500.155 seconds	419.292 seconds

In a full cache hit scenario, GCC 9 LTO workflow takes 20-30 minutes, while Clang 6 workflow can complete around 10 minutes. It is attractive to make Clang 6 the basic build test configuration.

Describe the solution

Shuffle matrix.yml, move Clang 6 to the front and rename its title to Basic Build Test. Repository branch protection rules also need to be updated to make the new title the required test.

Describe alternatives you've considered

Testing

Additional context