glsl: add a is_implicit_initializer flag

glsl: add a is_implicit_initializer flag

Shared globals and glsl_zero_init can cause linker errors if the
variable is only initialized in 1 place.

This commit adds a flag to variables that have been implicitely
initialized to be able in this situation to keep the explicit
initialization value.

Without this change the global-single-initializer-2-shaders piglit
test fails when using glsl_zero_init.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4607>