[libc++] Fixes the Clang modular build.

[Mordante]

Summary

The patch
430b397f6715d4d2f2569716c93fcd6292ace3dc ("[libc++] Granularize <type_traits> includes in <iterator>") missed some exports in the modulemap with became apparent in the reverting commit 12cb1cb3720d.

This fixes the issue by updating the modulemap. Note that the Clang ICE is not fixed by this change.

[Mordante]

libcxx/include/module.modulemap.in

1421

Note this looks wrong from libc++'s perspective. When this "solves" the issue we need to investigate why it fixes it; I think the export belongs to a different header.

[Mordante]

Fix CI run instead of libc++.

[Mordante]

Rebased and improve fixes.

[Mordante]

Finalized patch.

[Mordante]

Removes uninteded include.

[Mordante]

Undo CI changes.

[Mordante]

https://buildkite.com/llvm-project/libcxx-ci/builds/18148 shows this patch fixes the Clang modular build.

libcxx/include/module.modulemap.in

module type_traits {
     module copy_cvref                          { private header "__type_traits/copy_cvref.h" }
     module decay                               { private header "__type_traits/decay.h" }
     module dependent_type                      { private header "__type_traits/dependent_type.h" }
     module disjunction                         { private header "__type_traits/disjunction.h" }
     module enable_if                           { private header "__type_traits/enable_if.h" }
     module extent                              { private header "__type_traits/extent.h" }
     module has_unique_object_representation    { private header "__type_traits/has_unique_object_representation.h" }
     module has_virtual_destructor              { private header "__type_traits/has_virtual_destructor.h" }
-    module integral_constant                   { private header "__type_traits/integral_constant.h" }
+    module integral_constant                   {
+      private header "__type_traits/integral_constant.h"
+      export type_traits.common_reference
+    }
     module is_abstract                         { private header "__type_traits/is_abstract.h" }
     module is_aggregate                        { private header "__type_traits/is_aggregate.h" }
     module is_allocator                        { private header "__type_traits/is_allocator.h" }
     module is_always_bitcastable               { private header "__type_traits/is_always_bitcastable.h" }
     module is_arithmetic {
       private header "__type_traits/is_arithmetic.h"
       export integral_constant
     }

libcxx/utils/ci/generate-buildkite-pipeline

 if git diff --name-only HEAD~1 | grep -q -E "^libcxx/|^libcxxabi/|^libunwind/|^runtimes/|^cmake/"; then
   LIBCXX_CHANGED=true
 fi
 
 if git diff --name-only HEAD~1 | grep -q -E "^clang/"; then
   CLANG_CHANGED=true
 fi
 
-if [[ "${CLANG_CHANGED}" == "true" && "${LIBCXX_CHANGED}" != "true" ]]; then
+if [[ "${CLANG_CHANGED}" == "true" ]]; then
   cat libcxx/utils/ci/buildkite-pipeline-clang.yml
 else
   cat libcxx/utils/ci/buildkite-pipeline.yml
 fi