compiler/types: Don't store a mem_ctx per type

compiler/types: Don't store a mem_ctx per type

These are used only by types created at runtime.  Since those will follow
the lifetime of the glsl_type_cache, we can use its mem_ctx for all the types.

Without a mem_ctx, there's nothing to be done in the destructor, so remove it.

Note some keys are calculated by building a mock type, so we need to create
a tmp_ctx in some cases.  We'll get rid of them in a later commit.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25006>