[DebugInfo] Forward-declare PDBFile (NFC)

NativeEnumInjectedSources.h needs PDBFile but relies on a
forward declaration of PDBFile in InjectedSourceStream.h.
This patch adds a forward declaration right in
NativeEnumInjectedSources.h.

While we are at it, this patch removes the one in
InjectedSourceStream.h, where it is unnecessary.