fix(compiler-cli): better detect classes that are indirectly exported

[alxhub]

The compiler flag compileNonExportedClasses allows the Angular compiler to
process classes which are not exported at the top level of a source file.
This is often used to allow for AOT compilation of test classes inside
it() test blocks, for example.

Previously, the compiler would identify exported classes by looking for an
export modifier on the class declaration itself. This works for the
trivial case, but fails for indirectly exported classes:

// Component is declared unexported.
@Component({...})
class FooCmp {...}

// Indirect export of FooCmp
export {FooCmp};

This is not an immediate problem for most application builds, since the
default value for compileNonExportedClasses is true and therefore such
classes get compiled regardless.

However, in the Angular Language Service now, compileNonExportedClasses is
forcibly overridden to false. That's because the tsconfig used by the IDE
and Language Service is often far broader than the application build's
configuration, and pulls in test files that can contain unexported classes
not designed with AOT compilation in mind.

Therefore, the Language Service has trouble working with such structures.

In this commit, the ReflectionHost gains a new API for detecting whether a
class is exported. The implementation of this method now not only considers
the export modifier, but also scans the ts.SourceFile for indirect
exports like the example above. This ensures the above case will be
processed directly in the Language Service.

This new operation is cached using an expando symbol on the ts.SourceFile,
ensuring good performance even when scanning large source files with lots of
exports (e.g. a FESM file under ngcc).

Fixes #42184.

[atscott]

presubmit

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