[Clang][NFC] Refactor Targets.h to make it publicly accessible (#116090)

This PR is motivated by the requirements of ClangIR, which includes
compilation pipelines that do not always start from the Clang driver. In
these cases, accessing some target-specific information, such as
obtaining a data layout string for a given target triple or querying
other target details, requires foundational infrastructure like
`clang::TargetInfo`. Since ClangIR is actively being upstreamed, sharing
this logic across components has become essential, which leads to this
PR.

The function `clang::targets::AllocateTarget` serves as the factory for
Clang's `TargetInfo`. To enable sharing, this PR moves `AllocateTarget`
to a public header.

The existing header `clang/lib/Basic/Targets.h` previously contained two
parts: the `AllocateTarget` function and target-specific macro helpers.
With `AllocateTarget` moved, only the macro stuff remain in `Targets.h`.
To better organize the code, the macro helpers have been relocated to a
new file, `clang/lib/Basic/TargetDefines.h` (essentially a rename). The
original `Targets.h` now serves as a proxy header that includes both
headers to maintain compatibility.

Author: seven-mile

clang/include/clang/Basic/TargetInfo.h

@@ -1871,6 +1871,11 @@ class TargetInfo : public TransferrableTargetInfo,
   void CheckFixedPointBits() const;
 };
 
+namespace targets {
+std::unique_ptr<clang::TargetInfo>
+AllocateTarget(const llvm::Triple &Triple, const clang::TargetOptions &Opts);
+} // namespace targets
+
 }  // end namespace clang
 
 #endif

clang/lib/Basic/TargetDefines.h

@@ -0,0 +1,39 @@
+//===------- TargetDefines.h - Target define helpers ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares a series of helper functions for defining target-specific
+// macros.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_LIB_BASIC_TARGETDEFINES_H
+#define LLVM_CLANG_LIB_BASIC_TARGETDEFINES_H
+
+#include "clang/Basic/LangOptions.h"
+#include "clang/Basic/MacroBuilder.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace clang {
+namespace targets {
+/// Define a macro name and standard variants.  For example if MacroName is
+/// "unix", then this will define "__unix", "__unix__", and "unix" when in GNU
+/// mode.
+LLVM_LIBRARY_VISIBILITY
+void DefineStd(clang::MacroBuilder &Builder, llvm::StringRef MacroName,
+               const clang::LangOptions &Opts);
+
+LLVM_LIBRARY_VISIBILITY
+void defineCPUMacros(clang::MacroBuilder &Builder, llvm::StringRef CPUName,
+                     bool Tuning = true);
+
+LLVM_LIBRARY_VISIBILITY
+void addCygMingDefines(const clang::LangOptions &Opts,
+                       clang::MacroBuilder &Builder);
+} // namespace targets
+} // namespace clang
+#endif // LLVM_CLANG_LIB_BASIC_TARGETDEFINES_H

clang/lib/Basic/Targets.h

@@ -15,32 +15,7 @@
 #ifndef LLVM_CLANG_LIB_BASIC_TARGETS_H
 #define LLVM_CLANG_LIB_BASIC_TARGETS_H
 
-#include "clang/Basic/LangOptions.h"
-#include "clang/Basic/MacroBuilder.h"
+#include "TargetDefines.h"
 #include "clang/Basic/TargetInfo.h"
-#include "llvm/ADT/StringRef.h"
 
-namespace clang {
-namespace targets {
-
-LLVM_LIBRARY_VISIBILITY
-std::unique_ptr<clang::TargetInfo>
-AllocateTarget(const llvm::Triple &Triple, const clang::TargetOptions &Opts);
-
-/// DefineStd - Define a macro name and standard variants.  For example if
-/// MacroName is "unix", then this will define "__unix", "__unix__", and "unix"
-/// when in GNU mode.
-LLVM_LIBRARY_VISIBILITY
-void DefineStd(clang::MacroBuilder &Builder, llvm::StringRef MacroName,
-               const clang::LangOptions &Opts);
-
-LLVM_LIBRARY_VISIBILITY
-void defineCPUMacros(clang::MacroBuilder &Builder, llvm::StringRef CPUName,
-                     bool Tuning = true);
-
-LLVM_LIBRARY_VISIBILITY
-void addCygMingDefines(const clang::LangOptions &Opts,
-                       clang::MacroBuilder &Builder);
-} // namespace targets
-} // namespace clang
 #endif // LLVM_CLANG_LIB_BASIC_TARGETS_H