[clang] [Clang][Hexagon] Define _GNU_SOURCE when using picolibc in C++ mode (PR #201599)

[Fangrui Song via cfe-commits]

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@@ -813,6 +815,13 @@ void HexagonToolChain::addClangTargetOptions(const ArgList &DriverArgs,
     CC1Args.push_back("-mllvm");
     CC1Args.push_back("-hexagon-autohvx");
   }
+
+  // picolibc guards GNU-extension functions (e.g. strtold_l) with __GNU_VISIBLE
+  // which requires _GNU_SOURCE. Define it automatically for C++ compilations
+  // so that building libcxx against picolibc works without -D_GNU_SOURCE.
+  if (GetCStdlibType(DriverArgs) == ToolChain::CST_Picolibc &&
+      getDriver().CCCIsCXX())
+    CC1Args.push_back("-D_GNU_SOURCE");
 }
----------------
MaskRay wrote:

Such macro definitions should be moved to clang/lib/Basic/Targets/OSTargets.h from clangDriver. g++ defines `_GNU_SOURCE` for C++ mode.

Defining different macros depending on `GetCStdlibType` is not a good idea... 

https://github.com/llvm/llvm-project/pull/201599