[clang] [Clang][Driver] Merge the different strategies of how libc++ is included (PR #83721)

[Fangrui Song via cfe-commits]

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@@ -705,6 +705,47 @@ class ToolChain {
   AddClangCXXStdlibIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                                llvm::opt::ArgStringList &CC1Args) const;
 
+  struct IncludeStrategy {
+    enum AvailabilityOptions {
+      // Check whether the directory is exists before adding it to the
+      // include path. This is the case if AssumeAvailable isn't set.
+      CheckIfAvailable,
+
+      // Don't check whether the directory exists. Just assume it does and add
+      // the include.
+      AssumeAvailable,
+
+      // Use v<MaxNumber> that is inside `<IncludeRoot>/c++`. If not set, always
+      // uses v1.
+      UseMaxVersionAvailable,
+    };
+
+    IncludeStrategy(AvailabilityOptions Availability,
+                    bool AddTargetDirIfAvailable = false,
+                    bool PrintDebugStatements = false)
+        : Availability(Availability),
+          AddTargetDirIfAvailable(AddTargetDirIfAvailable),
+          PrintDebugStatements(PrintDebugStatements) {}
+
+    LLVM_PREFERRED_TYPE(AvailabilityOptions)
+    unsigned Availability : 2;
+
+    // Check whether the directory `<IncludeRoot>/<target-triple>/c++/v<N>`
+    // exists, and add it to the include path if it does.
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned AddTargetDirIfAvailable : 1;
+
+    // Whether to print a message if a checked directory isn't available.
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned PrintDebugStatements : 1;
+  };
+
+  /// Helper function to implement AddClangCXXStdlibIncludeArgs for libc++.
+  bool AddLibcxxInclude(const llvm::opt::ArgList &DriverArgs,
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MaskRay wrote:

The case of the first function name character is unfortunately inconsistent, but the convention is that newer function names look like `functionName`, e.g. addLibStdCxxIncludePaths, addGCCLibStdCxxIncludePaths

https://github.com/llvm/llvm-project/pull/83721