llvmPackages_10: rc2 -> rc3

http://lists.llvm.org/pipermail/llvm-dev/2020-March/139729.html

Additionally cherry-picked 3 commits from `llvm-project/master`:
- llvm/llvm-project@d21664c
- llvm/llvm-project@3a0f6e6
- llvm/llvm-project@87dac7d

such that clang can automatically pick up the polly plugin from the
`llvm-polly` build.

pkgs/development/compilers/llvm/10/clang/clang-extension-handling.patch

@@ -0,0 +1,18 @@
+Compressed diff from
+```
+git show d21664cce1db8debe2528f36b1fbd2b8af9c9401 87dac7da68ea1e0adac78c59ef1891dcf9632b67 3a0f6e699bb6d96dc62dce6faef20ac26cf103fd
+```
+with the purpose of avoiding linker errors arising in the polly-flavoured clang.
+
+diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
+index 781c3eb7f2f..dc1413f4b59 100644
+--- clang/CMakeLists.txt
++++ clang/CMakeLists.txt
+@@ -864,6 +864,7 @@ add_subdirectory(utils/hmaptool)
+ 
+ if(CLANG_BUILT_STANDALONE)
+   llvm_distribution_add_targets()
++  process_llvm_pass_plugins()
+ endif()
+ 
+ configure_file(

pkgs/development/compilers/llvm/10/clang/default.nix

@@ -1,7 +1,6 @@
 { stdenv, fetch, cmake, libxml2, llvm, version, clang-tools-extra_src, python3, lld
 , fixDarwinDylibNames
 , enableManpages ? false
-, enablePolly ? false # TODO: get this info from llvm (passthru?)
 }:
 
 let
@@ -9,7 +8,7 @@ let
     pname = "clang";
     inherit version;
 
-    src = fetch "clang" "1npwv0j6812q9jar79bb5m2j4lmvp11680in45nlma8czrs52w0v";
+    src = fetch "clang" "1w7ixr16a9f0g5kv4irvhwq973wn0d418kb0p9rabyfscm05wfmq";
 
     unpackPhase = ''
       unpackFile $src
@@ -34,12 +33,12 @@ let
       "-DSPHINX_OUTPUT_MAN=ON"
       "-DSPHINX_OUTPUT_HTML=OFF"
       "-DSPHINX_WARNINGS_AS_ERRORS=OFF"
-    ] ++ stdenv.lib.optionals enablePolly [
-      "-DWITH_POLLY=ON"
-      "-DLINK_POLLY_INTO_TOOLS=ON"
     ];
 
     patches = [
+      # 10.0.0rc3-only
+      ./clang-extension-handling.patch
+
       ./purity.patch
       # https://reviews.llvm.org/D51899
       ./compiler-rt-baremetal.patch