stage2: `zig test` now works with the LLVM backend

Frontend improvements:

 * When compiling in `zig test` mode, put a task on the work queue to
   analyze the main package root file. Normally, start code does
   `_ = import("root");` to make Zig analyze the user's code, however in
   the case of `zig test`, the root source file is the test runner.
   Without this change, no tests are picked up.
 * In the main pipeline, once semantic analysis is finished, if there
   are no compile errors, populate the `test_functions` Decl with the
   set of test functions picked up from semantic analysis.
 * Value: add `array` and `slice` Tags.

LLVM backend improvements:

 * Fix incremental updates of globals. Previously the
   value of a global would not get replaced with a new value.
 * Fix LLVM type of arrays. They were incorrectly sending
   the ABI size as the element count.
 * Remove the FuncGen parameter from genTypedValue. This function is for
   generating global constants and there is no function available when
   it is being called.
   - The `ref_val` case is now commented out. I'd like to eliminate
     `ref_val` as one of the possible Value Tags. Instead it should
     always be done via `decl_ref`.
 * Implement constant value generation for slices, arrays, and structs.
 * Constant value generation for functions supports the `decl_ref` tag.

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