[clang] Remove unused forward declarations (NFC)

Author: kazutakahirata

clang/include/clang/Serialization/ModuleManager.h

@@ -39,7 +39,6 @@ class FileManager;
 class GlobalModuleIndex;
 class HeaderSearch;
 class InMemoryModuleCache;
-class ModuleMap;
 class PCHContainerReader;
 
 namespace serialization {

clang/lib/AST/Interp/EvalEmitter.h

@@ -23,7 +23,6 @@
 #include "llvm/Support/Error.h"
 
 namespace clang {
-class FunctionDecl;
 namespace interp {
 class Context;
 class Function;

clang/lib/CodeGen/CGHLSLRuntime.h

@@ -37,7 +37,6 @@ class VarDecl;
 class ParmVarDecl;
 class HLSLBufferDecl;
 class HLSLResourceBindingAttr;
-class CallExpr;
 class Type;
 class DeclContext;