llvm-polly,clang-polly: Add llvm/clang variants w/polly support (#55065)

* llvm: build w/polly in-tree, optionally

Don't enable by default yet, defer rebuilds.

* top-level: llvm-polly, clang-polly

* bit hacky re:overrides
* need to explicitly set cmake flags for clang to link in polly

pkgs/development/compilers/llvm/7/clang/default.nix

@@ -1,6 +1,7 @@
 { stdenv, fetch, cmake, libxml2, llvm, version, clang-tools-extra_src, python
 , fixDarwinDylibNames
 , enableManpages ? false
+, enablePolly ? false # TODO: get this info from llvm (passthru?)
 }:
 
 let
@@ -30,6 +31,9 @@ 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 = [ ./purity.patch ];