[ci] Write test results to unique file names

In this patch I'm using a new lit option so that the pipeline
writes many results files, one for each time lit is run:
```
--use-unique-output-file-name
  When enabled, lit will add a unique element to the output file name, before the extension. For example "results.xml" will become "results.<something>.xml". The
  "<something>" is not ordered in any way and is chosen so that existing files are not overwritten. [Default: Off]
```

(I added this to lit recently)

Now if I run the Linux build:
$ bash ./.ci/monolithic-linux.sh "clang;lldb;lld" "check-lldb-shell check-lld" "libcxx;libcxxabi" "check-libcxx check-libcxxabi"

I get multiple test result files. In my case some tests fail so runtimes aren't checked, but all projects are
so there is 1 file for lldb and one for lld:
$ ls build/*.xml
build/test-results.klc82utf.xml  build/test-results.majylh73.xml

This change just collects the XML files as artifacts. Once I know that's
working, I can setup a test reporting plugin to build a summary from them.

Author: DavidSpickett

.ci/generate-buildkite-pipeline-premerge

@@ -272,7 +272,7 @@ if [[ "${linux_projects}" != "" ]]; then
   artifact_paths:
   - 'artifacts/**/*'
   - '*_result.json'
-  - 'build/test-results.xml'
+  - 'build/test-results.*.xml'
   agents: ${LINUX_AGENTS}
   retry:
     automatic:
@@ -295,7 +295,7 @@ if [[ "${windows_projects}" != "" ]]; then
   artifact_paths:
   - 'artifacts/**/*'
   - '*_result.json'
-  - 'build/test-results.xml'
+  - 'build/test-results.*.xml'
   agents: ${WINDOWS_AGENTS}
   retry:
     automatic:

.ci/monolithic-linux.sh

@@ -37,6 +37,8 @@ trap show-stats EXIT
 projects="${1}"
 targets="${2}"
 
+lit_args="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml --use-unique-output-file-name --timeout=1200 --time-tests"
+
 echo "--- cmake"
 pip install -q -r "${MONOREPO_ROOT}"/mlir/python/requirements.txt
 pip install -q -r "${MONOREPO_ROOT}"/lldb/test/requirements.txt
@@ -47,7 +49,7 @@ cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D LLVM_ENABLE_ASSERTIONS=ON \
       -D LLVM_BUILD_EXAMPLES=ON \
       -D COMPILER_RT_BUILD_LIBFUZZER=OFF \
-      -D LLVM_LIT_ARGS="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml --timeout=1200 --time-tests" \
+      -D LLVM_LIT_ARGS="${lit_args}" \
       -D LLVM_ENABLE_LLD=ON \
       -D CMAKE_CXX_FLAGS=-gmlt \
       -D LLVM_CCACHE_BUILD=ON \
@@ -87,7 +89,8 @@ if [[ "${runtimes}" != "" ]]; then
       -D CMAKE_BUILD_TYPE=RelWithDebInfo \
       -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
       -D LIBCXX_TEST_PARAMS="std=c++03" \
-      -D LIBCXXABI_TEST_PARAMS="std=c++03"
+      -D LIBCXXABI_TEST_PARAMS="std=c++03" \
+      -D LLVM_LIT_ARGS="${lit_args}"
 
   echo "--- ninja runtimes C++03"
 
@@ -104,7 +107,8 @@ if [[ "${runtimes}" != "" ]]; then
       -D CMAKE_BUILD_TYPE=RelWithDebInfo \
       -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
       -D LIBCXX_TEST_PARAMS="std=c++26" \
-      -D LIBCXXABI_TEST_PARAMS="std=c++26"
+      -D LIBCXXABI_TEST_PARAMS="std=c++26" \
+      -D LLVM_LIT_ARGS="${lit_args}"
 
   echo "--- ninja runtimes C++26"
 
@@ -121,7 +125,8 @@ if [[ "${runtimes}" != "" ]]; then
       -D CMAKE_BUILD_TYPE=RelWithDebInfo \
       -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
       -D LIBCXX_TEST_PARAMS="enable_modules=clang" \
-      -D LIBCXXABI_TEST_PARAMS="enable_modules=clang"
+      -D LIBCXXABI_TEST_PARAMS="enable_modules=clang" \
+      -D LLVM_LIT_ARGS="${lit_args}"
 
   echo "--- ninja runtimes clang modules"
 

.ci/monolithic-windows.sh

@@ -53,7 +53,7 @@ cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D LLVM_ENABLE_ASSERTIONS=ON \
       -D LLVM_BUILD_EXAMPLES=ON \
       -D COMPILER_RT_BUILD_LIBFUZZER=OFF \
-      -D LLVM_LIT_ARGS="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml --timeout=1200 --time-tests" \
+      -D LLVM_LIT_ARGS="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml --use-unique-output-file-name --timeout=1200 --time-tests" \
       -D COMPILER_RT_BUILD_ORC=OFF \
       -D CMAKE_C_COMPILER_LAUNCHER=sccache \
       -D CMAKE_CXX_COMPILER_LAUNCHER=sccache \