[CodeGen] Remove unused includes (NFC) (llvm#141418)

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Author: kazutakahirata

clang/lib/CodeGen/BackendUtil.cpp

@@ -16,7 +16,6 @@
 #include "clang/Frontend/FrontendDiagnostic.h"
 #include "clang/Frontend/Utils.h"
 #include "clang/Lex/HeaderSearchOptions.h"
-#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Analysis/GlobalsModRef.h"

clang/lib/CodeGen/CGBuiltin.cpp

@@ -26,9 +26,7 @@
 #include "TargetInfo.h"
 #include "clang/AST/OSLog.h"
 #include "clang/AST/StmtVisitor.h"
-#include "clang/Basic/TargetBuiltins.h"
 #include "clang/Basic/TargetInfo.h"
-#include "clang/Basic/TargetOptions.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
 #include "llvm/IR/InlineAsm.h"
 #include "llvm/IR/Intrinsics.h"

clang/lib/CodeGen/Targets/AMDGPU.cpp

@@ -8,7 +8,6 @@
 
 #include "ABIInfoImpl.h"
 #include "TargetInfo.h"
-#include "clang/Basic/TargetOptions.h"
 #include "llvm/Support/AMDGPUAddrSpace.h"
 
 using namespace clang;