[CI] Increase Configurability of Monolithic Windows Build (#124328)

This patch makes it so that the caller of monolithic-windows.sh can set
the maximum number of parallel compile/link jobs in an environment
variable rather than manually specifying it inside of the CMake.
Additionally, the env variable definitions for CC, CXX, and LD are sunk
into the shell script due to those config options being pretty inherent
to what the pipeline is testing.

This is intended to make things more flexible/useable for the new
premerge CI pipeline, particularly as we are looking at using larger
runners and want the increased flexibility to experiment.

Author: boomanaiden154

.ci/generate-buildkite-pipeline-premerge

@@ -128,9 +128,8 @@ if [[ "${windows_projects}" != "" ]]; then
         limit: 2
   timeout_in_minutes: 150
   env:
-    CC: 'cl'
-    CXX: 'cl'
-    LD: 'link'
+    MAX_PARALLEL_COMPILE_JOBS: '16'
+    MAX_PARALLEL_LINK_JOBS: '4'
   commands:
   - 'C:\\BuildTools\\Common7\\Tools\\VsDevCmd.bat -arch=amd64 -host_arch=amd64'
   - 'bash .ci/monolithic-windows.sh "$(echo ${windows_projects} | tr ' ' ';')" "$(echo ${windows_check_targets})"'

.ci/monolithic-windows.sh

@@ -50,6 +50,10 @@ echo "--- cmake"
 pip install -q -r "${MONOREPO_ROOT}"/mlir/python/requirements.txt
 pip install -q -r "${MONOREPO_ROOT}"/.ci/requirements.txt
 
+export CC=cl
+export CXX=cl
+export LD=link
+
 # The CMAKE_*_LINKER_FLAGS to disable the manifest come from research
 # on fixing a build reliability issue on the build server, please
 # see https://github.com/llvm/llvm-project/pull/82393 and
@@ -72,8 +76,8 @@ cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D CMAKE_EXE_LINKER_FLAGS="/MANIFEST:NO" \
       -D CMAKE_MODULE_LINKER_FLAGS="/MANIFEST:NO" \
       -D CMAKE_SHARED_LINKER_FLAGS="/MANIFEST:NO" \
-      -D LLVM_PARALLEL_COMPILE_JOBS=16 \
-      -D LLVM_PARALLEL_LINK_JOBS=4
+      -D LLVM_PARALLEL_COMPILE_JOBS=${MAX_PARALLEL_COMPILE_JOBS} \
+      -D LLVM_PARALLEL_LINK_JOBS=${MAX_PARALLEL_LINK_JOBS}
 
 echo "--- ninja"
 # Targets are not escaped as they are passed as separate arguments.