clang_4: fix build w/gcc9

As noted in the incorporated patch, name collisions between lambda
arguments and lambda captures cause compilation failures in GCC9[1].

The issue has been fixed in a later version of CLang, unfortunately the
relevant patches weren't backported to the versions 3.9 and 4.0[2].

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2211
[2] bb177a9307 (diff-d63bede6ed85300ad2aaf4eaee571456L6278)

pkgs/development/compilers/llvm/4/clang/0001-Fix-compilation-w-gcc9.patch

@@ -0,0 +1,59 @@
+From a2af0b02eba35d0670e3e442ff7c61b3e2304edd Mon Sep 17 00:00:00 2001
+From: Maximilian Bosch <maximilian@mbosch.me>
+Date: Mon, 30 Dec 2019 02:11:35 +0100
+Subject: [PATCH] Fix compilation w/gcc9
+
+Build broken with the following errors:
+
+```
+clang> /build/clang/lib/CodeGen/CGOpenMPRuntime.cpp: In lambda function:
+clang> /build/clang/lib/CodeGen/CGOpenMPRuntime.cpp:6275:24: error: lambda parameter 'CGF' previously declared as a capture
+clang> 6275 |       CodeGenFunction &CGF, PrePostActionTy &) {
+clang> |       ~~~~~~~~~~~~~~~~~^~~
+clang> /build/clang/lib/CodeGen/CGOpenMPRuntime.cpp: In lambda function:
+clang> /build/clang/lib/CodeGen/CGOpenMPRuntime.cpp:6321:62: error: lambda parameter 'CGF' previously declared as a capture
+clang> 6321 |   auto &&EndThenGen = [&CGF, Device, &Info](CodeGenFunction &CGF,
+clang> |                                             ~~~~~~~~~~~~~~~~~^~~
+clang> /build/clang/lib/CodeGen/CGOpenMPRuntime.cpp: In lambda function:
+clang> /build/clang/lib/CodeGen/CGOpenMPRuntime.cpp:6400:56: error: lambda parameter 'CGF' previously declared as a capture
+clang> 6400 |   auto &&ThenGen = [&D, &CGF, Device](CodeGenFunction &CGF, PrePostActionTy &) {
+clang> |                                       ~~~~~~~~~~~~~~~~~^~~
+```
+---
+ lib/CodeGen/CGOpenMPRuntime.cpp | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/lib/CodeGen/CGOpenMPRuntime.cpp b/lib/CodeGen/CGOpenMPRuntime.cpp
+index 4025217..40a73ef 100644
+--- a/lib/CodeGen/CGOpenMPRuntime.cpp
++++ b/lib/CodeGen/CGOpenMPRuntime.cpp
+@@ -6271,7 +6271,7 @@ void CGOpenMPRuntime::emitTargetDataCalls(
+   // Generate the code for the opening of the data environment. Capture all the
+   // arguments of the runtime call by reference because they are used in the
+   // closing of the region.
+-  auto &&BeginThenGen = [&D, &CGF, Device, &Info, &CodeGen, &NoPrivAction](
++  auto &&BeginThenGen = [&D, Device, &Info, &CodeGen, &NoPrivAction](
+       CodeGenFunction &CGF, PrePostActionTy &) {
+     // Fill up the arrays with all the mapped variables.
+     MappableExprsHandler::MapBaseValuesArrayTy BasePointers;
+@@ -6318,7 +6318,7 @@ void CGOpenMPRuntime::emitTargetDataCalls(
+   };
+ 
+   // Generate code for the closing of the data region.
+-  auto &&EndThenGen = [&CGF, Device, &Info](CodeGenFunction &CGF,
++  auto &&EndThenGen = [Device, &Info](CodeGenFunction &CGF,
+                                             PrePostActionTy &) {
+     assert(Info.isValid() && "Invalid data environment closing arguments.");
+ 
+@@ -6397,7 +6397,7 @@ void CGOpenMPRuntime::emitTargetDataStandAloneCall(
+          "Expecting either target enter, exit data, or update directives.");
+ 
+   // Generate the code for the opening of the data environment.
+-  auto &&ThenGen = [&D, &CGF, Device](CodeGenFunction &CGF, PrePostActionTy &) {
++  auto &&ThenGen = [&D, Device](CodeGenFunction &CGF, PrePostActionTy &) {
+     // Fill up the arrays with all the mapped variables.
+     MappableExprsHandler::MapBaseValuesArrayTy BasePointers;
+     MappableExprsHandler::MapValuesArrayTy Pointers;
+-- 
+2.23.1
+

pkgs/development/compilers/llvm/4/clang/default.nix

@@ -38,7 +38,10 @@ let
     ++ stdenv.lib.optional stdenv.isLinux "-DGCC_INSTALL_PREFIX=${gcc}"
     ++ stdenv.lib.optional (stdenv.cc.libc != null) "-DC_INCLUDE_DIRS=${stdenv.cc.libc}/include";
 
-    patches = [ ./purity.patch ];
+    patches = [
+      ./purity.patch
+      ./0001-Fix-compilation-w-gcc9.patch
+    ];
 
     postPatch = ''
       sed -i -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' lib/Driver/Tools.cpp