swiftPackages.swift-unwrapped: vendor Clang patch

The versions of `llvmPackages` that require this patch are going
away soon.

pkgs/development/compilers/swift/compiler/default.nix

···
 
      patch -p1 -d llvm-project/clang -i ${./patches/clang-toolchain-dir.patch}
      patch -p1 -d llvm-project/clang -i ${./patches/clang-wrap.patch}
-     patch -p1 -d llvm-project/clang -i ${../../llvm/12/clang/purity.patch}
+     patch -p1 -d llvm-project/clang -i ${./patches/clang-purity.patch}
      patch -p2 -d llvm-project/clang -i ${
        fetchpatch {
          name = "clang-cmake-fix-interpreter.patch";

pkgs/development/compilers/swift/compiler/patches/clang-purity.patch

···
+From 4add81bba40dcec62c4ea4481be8e35ac53e89d8 Mon Sep 17 00:00:00 2001
+From: Will Dietz <[e-mail]>
+Date: Thu, 18 May 2017 11:56:12 -0500
+Subject: [PATCH] "purity" patch for 5.0
+
+---
+ lib/Driver/ToolChains/Gnu.cpp | 7 -------
+ 1 file changed, 7 deletions(-)
+
+diff --git a/lib/Driver/ToolChains/Gnu.cpp b/lib/Driver/ToolChains/Gnu.cpp
+index fe3c0191bb..c6a482bece 100644
+--- a/lib/Driver/ToolChains/Gnu.cpp
++++ b/lib/Driver/ToolChains/Gnu.cpp
+@@ -487,12 +487,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
+   if (!IsStatic) {
+     if (Args.hasArg(options::OPT_rdynamic))
+       CmdArgs.push_back("-export-dynamic");
+-
+-    if (!Args.hasArg(options::OPT_shared) && !IsStaticPIE) {
+-      CmdArgs.push_back("-dynamic-linker");
+-      CmdArgs.push_back(Args.MakeArgString(Twine(D.DyldPrefix) +
+-                                           ToolChain.getDynamicLinker(Args)));
+-    }
+   }
+ 
+   CmdArgs.push_back("-o");
+-- 
+2.11.0