[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.

Author: kazutakahirata

llvm/include/llvm/DebugInfo/PDB/Native/InjectedSourceStream.h

@@ -18,7 +18,6 @@ namespace msf {
 class MappedBlockStream;
 }
 namespace pdb {
-class PDBFile;
 class PDBStringTable;
 
 class InjectedSourceStream {

llvm/include/llvm/DebugInfo/PDB/Native/NativeEnumInjectedSources.h

@@ -17,6 +17,7 @@ namespace llvm {
 namespace pdb {
 
 class InjectedSourceStream;
+class PDBFile;
 class PDBStringTable;
 
 class NativeEnumInjectedSources : public IPDBEnumChildren<IPDBInjectedSource> {