LLVM_ENABLE_RUNTIMES=flang-rt for premerge-monolithic-* (#396)

Update premerge-monolithic-windows and premerge-monolithic-linux to
prepare for the removal of the "projects" build of the flang runtime in
llvm/llvm-project#124126.

This does not change the actual premerge GitHub action, which is done by
llvm/llvm-project#128678

For premerge-monolithic-linux, add flang-rt to the LLVM_ENABLE_RUNTIMES
list, indirectly by adding it to `depends_on_projects` which also
updated the build scheduler.

For premerge-monolithic-windows, remove building flang to match the
actual pre-merge build which disabled building flang on Windows in
llvm/llvm-project@e4b424a.
Adding flang-rt would also require compiler-rt (which was always
required on Windows, but now there is a regression test for it) and
check-compiler-rt is currently failing.

Split off from #333. Verified to work locally using instruction from
https://llvm.org/docs/HowToAddABuilder.html#testing-a-builder-config-locally.
With the exception of `-gmlt` which seems to be a Google-only extention
of Clang.

Affected builders:
* [premerge-monolithic-windows](https://lab.llvm.org/buildbot/#/builders/35)
* [premerge-monolithic-linux](https://lab.llvm.org/buildbot/#/builders/153)

Affected workers:
 * [premerge-windows-1](https://lab.llvm.org/buildbot/#/workers/153)
 * [premerge-linux-1](https://lab.llvm.org/buildbot/#/workers/110)

buildbot/osuosl/master/config/builders.py

@@ -3359,7 +3359,7 @@
     'builddir': "premerge-monolithic-windows",
     'factory' : UnifiedTreeBuilder.getCmakeWithNinjaWithMSVCBuildFactory(
                     vs="autodetect",
-                    depends_on_projects=["clang-tools-extra", "clang", "flang", "libclc", "lld", "llvm", "mlir", "polly", "pstl"],
+                    depends_on_projects=["clang-tools-extra", "clang", "libclc", "lld", "llvm", "mlir", "polly", "pstl"],
                     checks=["check-all"],
                     install_pip_requirements = True,
                     extra_configure_args=[
@@ -3379,7 +3379,7 @@
     'workernames': ["premerge-linux-1"],
     'builddir': "premerge-monolithic-linux",
     'factory': UnifiedTreeBuilder.getCmakeWithNinjaBuildFactory(
-                    depends_on_projects=["bolt", "clang", "clang-tools-extra", "compiler-rt", "flang", "libc", "libclc", "lld", "llvm", "mlir", "polly", "pstl"],
+                    depends_on_projects=["bolt", "clang", "clang-tools-extra", "compiler-rt", "flang", "flang-rt", "libc", "libclc", "lld", "llvm", "mlir", "polly", "pstl"],
                     install_pip_requirements = True,
                     extra_configure_args=[
                       "-DCMAKE_BUILD_TYPE=Release",