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
