[lld] Add missing includes. (llvm#143453)

Some inline methods in these headers require a complete type but the
corresponding include was missing.

Author: slackito

lld/COFF/TypeMerger.h

@@ -9,6 +9,7 @@
 #ifndef LLD_COFF_TYPEMERGER_H
 #define LLD_COFF_TYPEMERGER_H
 
+#include "COFFLinkerContext.h"
 #include "Config.h"
 #include "DebugTypes.h"
 #include "lld/Common/Timer.h"

lld/ELF/DWARF.h

@@ -10,6 +10,7 @@
 #define LLD_ELF_DWARF_H
 
 #include "InputFiles.h"
+#include "InputSection.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/DebugInfo/DWARF/DWARFContext.h"