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/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