Build fixes for older GCC and Ubuntu

- Ubuntu 18.04 ships with GCC 7, which needs `std::move()` when returning an `absl::StatusOr<>`
- Ignore C++ AST nodes of type `cindex.TypeKind.UNEXPOSED` in Python generator
- Remove default values in `ubuntu-cmake.yml`

PiperOrigin-RevId: 347605109
Change-Id: Ibe167249ecf4ef1af1654d63c2e067fc02e5782d

.github/workflows/ubuntu-cmake.yml

@@ -20,21 +20,18 @@ jobs:
 
     - name: Create Build Environment
       run: |
-        pip3 install absl-py clang
+        pip3 install absl-py
         cmake -E make_directory ${{runner.workspace}}/build
 
     - name: Configure CMake
-      shell: bash
       working-directory: ${{runner.workspace}}/build
       run: cmake $GITHUB_WORKSPACE -G Ninja -DCMAKE_BUILD_TYPE=$BUILD_TYPE
 
     - name: Build
       working-directory: ${{runner.workspace}}/build
-      shell: bash
       run: cmake --build . --config $BUILD_TYPE
 
     # TODO(cblichmann): Before enabling this, make sure all OSS tests pass
     #- name: Test
     #  working-directory: ${{runner.workspace}}/build
-    #  shell: bash
     #  run: ctest -C $BUILD_TYPE

sandboxed_api/sandbox2/buffer.cc

@@ -49,7 +49,7 @@ absl::StatusOr<std::unique_ptr<Buffer>> Buffer::CreateFromFd(int fd) {
   }
   buffer->fd_ = fd;
   buffer->size_ = size;
-  return buffer;
+  return std::move(buffer);  // GCC 7 needs the move (C++ DR #1579)
 }
 
 // Creates a new Buffer of the specified size, backed by a temporary file that

sandboxed_api/tools/generator2/code.py

@@ -266,10 +266,10 @@ class Type(object):
       if not skip_self:
         result.add(self)
         self._tu.search_for_macro_name(self._get_declaration())
-
       return result
 
-    raise ValueError('Unhandled kind: {}'.format(self._clang_type.kind))
+    # Ignore all cindex.TypeKind.UNEXPOSED AST nodes
+    return result
 
   def _get_related_types_of_typedef(self, result):
     # type: (Set[Type]) -> Set[Type]