frontend: rework `@embedFile` for incremental compilation

This feature was made to work with the legacy incremental compilation
mechanism which is being reworked.

This commit regresses the ability to update files used with `@embedFile`
while the compiler is running.

In exchange, we get these benefits:
* The embedded file contents are copied directly into InternPool rather
  than there being an extra allocation and memcpy.
* The EmbedFile struct, which represents a long-lived object, is made
  more serialization friendly.
* Eliminate the creation and use of a Decl as an anonymous decl.

When implementing the new incremental compilation mechanism,
functionality will need to be added back for handling `@embedFile`.

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