llvm: provide fix for python dependencies (#20356)

Previously compiler-rt didn't correctly passthrough cmake
variables for python when building the various santizers.
This patch passes these variables through.

This patch may also correctly apply to any version of LLVM
to any version of LLVM that uses the newer monorepo style organization,
and any older llvm newer than 7.0.0 as long as the paths were set
appropriately.  However, this was not done so because it was not
tested with older LLVM releases.

Fixes #19908
See also: https://bugs.llvm.org/show_bug.cgi?id=48180

var/spack/repos/builtin/packages/llvm/llvm_python_path.patch

@@ -0,0 +1,14 @@
+diff --git a/compiler-rt/cmake/Modules/AddCompilerRT.cmake b/compiler-rt/cmake/Modules/AddCompilerRT.cmake
+index dab55707338..6f4c6791141 100644
+--- a/compiler-rt/cmake/Modules/AddCompilerRT.cmake
++++ b/compiler-rt/cmake/Modules/AddCompilerRT.cmake
+@@ -612,6 +612,9 @@ macro(add_custom_libcxx name prefix)
+     CMAKE_OBJDUMP
+     CMAKE_STRIP
+     CMAKE_SYSROOT
++    PYTHON_EXECUTABLE
++    Python3_EXECUTABLE
++    Python2_EXECUTABLE
+     CMAKE_SYSTEM_NAME)
+   foreach(variable ${PASSTHROUGH_VARIABLES})
+     get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)

var/spack/repos/builtin/packages/llvm/package.py

@@ -225,6 +225,10 @@ class Llvm(CMakePackage, CudaPackage):
     # merged in llvm-11.0.0_rc2
     patch("lldb_external_ncurses-10.patch", when="@10.0.0:10.99+lldb")
 
+    # https://github.com/spack/spack/issues/19908
+    # merged in llvm main prior to 12.0.0
+    patch("llvm_python_path.patch", when="@11.0.0")
+
     # The functions and attributes below implement external package
     # detection for LLVM. See:
     #
@@ -365,16 +369,24 @@ def setup_run_environment(self, env):
 
     def cmake_args(self):
         spec = self.spec
+        python = spec['python']
         cmake_args = [
             "-DLLVM_REQUIRES_RTTI:BOOL=ON",
             "-DLLVM_ENABLE_RTTI:BOOL=ON",
             "-DLLVM_ENABLE_EH:BOOL=ON",
             "-DCLANG_DEFAULT_OPENMP_RUNTIME:STRING=libomp",
-            "-DPYTHON_EXECUTABLE:PATH={0}".format(spec["python"].command.path),
+            "-DPYTHON_EXECUTABLE:PATH={0}".format(python.command.path),
             "-DLIBOMP_USE_HWLOC:BOOL=ON",
             "-DLIBOMP_HWLOC_INSTALL_DIR={0}".format(spec["hwloc"].prefix),
         ]
 
+        if python.version >= Version("3.0.0"):
+            cmake_args.append("-DPython3_EXECUTABLE={0}".format(
+                              python.command.path))
+        else:
+            cmake_args.append("-DPython2_EXECUTABLE={0}".format(
+                              python.command.path))
+
         projects = []
 
         if "+cuda" in spec: