[libc++][ci] Add a test configuration with an incomplete sysroot (#107089)

When bringing up a new cross compiler from scratch, we build
libunwind/libcxx in a setup where the toolchain is incomplete and unable
to perform the normal linker checks; this requires a few special cases
in the CMake files.

We simulate that scenario by removing the libc++ headers, libunwind and
libc++ libraries from the installed toolchain.

We need to set CMAKE_CXX_COMPILER_WORKS since CMake fails to probe the
compiler. We need to set CMAKE_CXX_COMPILER_TARGET, since LLVM's
heuristics fail when CMake hasn't been able to probe the environment
properly. (This is normal; one has to set those options when setting up
such a toolchain from scratch.)

This adds CI coverage for these build scenarios, which otherwise seldom
are tested by some build flow (but are essential when setting up a cross
compiler from scratch).

Author: mstorsjo

.github/workflows/libcxx-build-and-test.yaml

@@ -242,6 +242,7 @@ jobs:
         - { config: mingw-dll, mingw: true }
         - { config: mingw-static, mingw: true }
         - { config: mingw-dll-i686, mingw: true }
+        - { config: mingw-incomplete-sysroot, mingw: true }
     steps:
       - uses: actions/checkout@v4
       - name: Install dependencies
@@ -260,6 +261,12 @@ jobs:
           del llvm-mingw*.zip
           mv llvm-mingw* c:\llvm-mingw
           echo "c:\llvm-mingw\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
+      - name: Simulate a from-scratch build of llvm-mingw
+        if: ${{ matrix.config == 'mingw-incomplete-sysroot' }}
+        run: |
+          rm -r c:\llvm-mingw\include\c++
+          rm -r c:\llvm-mingw\*-w64-mingw32\lib\libc++*
+          rm -r c:\llvm-mingw\*-w64-mingw32\lib\libunwind*
       - name: Add Git Bash to the path
         run: |
           echo "c:\Program Files\Git\usr\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append

libcxx/utils/ci/run-buildbot

@@ -715,6 +715,29 @@ mingw-dll-i686)
           -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake"
     check-runtimes
 ;;
+mingw-incomplete-sysroot)
+    # When bringing up a new cross compiler from scratch, we build
+    # libunwind/libcxx in a setup where the toolchain is incomplete and
+    # unable to perform the normal linker checks; this requires a few
+    # special cases in the CMake files.
+    #
+    # Building in an incomplete setup requires setting CMAKE_*_COMPILER_WORKS,
+    # as CMake fails to probe the compiler. This case also requires
+    # setting CMAKE_CXX_COMPILER_TARGET, as LLVM's heuristics for setting
+    # the triple fails when CMake hasn't been able to probe the environment.
+    # (This is what one has to do when building the initial libunwind/libcxx
+    # for a new toolchain.)
+    clean
+    generate-cmake \
+          -DCMAKE_C_COMPILER_WORKS=TRUE \
+          -DCMAKE_CXX_COMPILER_WORKS=TRUE \
+          -DCMAKE_C_COMPILER_TARGET=x86_64-w64-windows-gnu \
+          -DCMAKE_CXX_COMPILER_TARGET=x86_64-w64-windows-gnu \
+          -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake"
+    # Only test that building succeeds; there's not much extra value in running
+    # the tests here, as it would be equivalent to the mingw-dll config above.
+    ${NINJA} -vC "${BUILD_DIR}"
+;;
 aix)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AIX.cmake" \