Introduce MetadataSource as a way to store the original descriptor for
any element (before any lowerings) and maintain it until the end of the
codegen where it's used in generating the metadata. Note that JVM
signatures written to the metadata are formed from the _resulting_
generated elements, not by mapping the original descriptors.
Some corner cases are not supported yet, namely properties declared in
companion objects, synthetic methods for property annotations,
JvmPackageName, etc.
#KT-29119 Fixed
IrDeclarationOrigin.FILE_CLASS is used in CallableReferenceLowering to
generate correct declaration owner.
Many reflection tests start to fail with this commit because they are
now treating callable references to top level declarations as Kotlin
symbols and fail because there's no JVM signature for them; this is
fixed in subsequent commits (previously, they worked because without
Kotlin metadata, these files were treated as Java classes)
