Include a "stack trace" with nested-import errors (#1872)

* Include a "stack trace" with nested-import errors

When errors occur in nested `#include` files it is often helpful to have a "stack trace" / traceback of the `#include` chain that led from a root translation unit to the file with an error.

This change implements a similar feature for `import`s.

It is worth noting that `import`s don't really *require* this kind of compiler support the way `#include`s do because the intention is that the meaning of an `import`ed file does not depend on the order or nesting of `import`s. As such, when trying to *fix* an error in an `import`ed file, you usually don't care how it came to be `import`ed into your shaders.

The use case here is somebody adapting a large body of Slang code to use in a different codebase, such that they have certain `.slang` files they don't actually intend to have compile correctly, and they want to be able to diagnose how they came to include those files when/if they cause problems.

The actual feature implementation is pretty simple because we already track a stack of active `import`s so that we can detect and diagnose recursive `import`s. This change simply changes the disagnostics when there is an error in imported code so that instead of just noting the inner-most `import` site it lists all the `import` sites that were active at the time.

The change includes a test case to confirm that the behavior works (at least for the case of a parse error).

* fixup: test outputs

Co-authored-by: Yong He <yonghe@outlook.com>
Co-authored-by: jsmall-nvidia <jsmall@nvidia.com>

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