WIP: [libc] Fix potential install errors related to missing headers

[stephenneuendorffer]

Summary

The current code seems to assume that any header required by a
particular target is also a dependency of libc.a. If this is not
the case, then the 'install' rule can break because it will try to
install a header file which has not, in fact, been generated.
This is really a fundamental problem with the way that
libc/include/CMakeList.txt manages the install header files and
targets, but there doesn't appear to be a good way to fix it in
current versions of cmake (which are limited in understanding the
interaction between file sets of custom targets and the install
rule).

[sivachandra]

We never tested this with a make install or ninja install. Would adding EXCLUDE_FROM_ALL here fix your problem: https://github.com/llvm/llvm-project/blob/main/libc/include/CMakeLists.txt#L520. The install-libc-headers target should set up the dependencies correctly and should still work with make install or ninja install: https://github.com/llvm/llvm-project/blob/main/libc/CMakeLists.txt#L215

libc/lib/CMakeLists.txt

DEPENDS
       ${${archive_2}}
   )
   set_target_properties(
     ${archive_1}
     PROPERTIES
       ARCHIVE_OUTPUT_NAME ${archive_0}
   )
   list(APPEND added_archive_targets ${archive_1})
+
+  # The dependency below is necessary because it's possible for libc-headers to generate
+  # and want to install headers that no component entrypoint depends on.
+  # This causes issues when running 'make install' because libc-headers is all implemented
+  # with custom targets and there is not good support for describing install dependencies
+  # on custom targets.
+  add_dependencies(${archive_1} libc-headers)
 endforeach()
 
 if(LIBC_TARGET_TRIPLE)
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LIBC_TARGET_TRIPLE})
 elseif(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT LIBC_GPU_BUILD)
   set(LIBC_INSTALL_LIBRARY_DIR
       lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE})
 else()