Visual 2019 Enterprise Preview
Visual Studio 2019 Tools Command Prompt for targeting x64

Instructions assume directories are already created.

To get code:

set PreferredToolArchitecture=x64
cd /D D:\src_builds\llvm-monorepo
git config --global protocol.version 2
git clone https://github.com/llvm/llvm-project.git llvm
cd /D D:\src_builds\llvm-monorepo\llvm
git config branch.master.rebase true

To update code:

set PreferredToolArchitecture=x64
cd /D D:\src_builds\llvm-monorepo\llvm
git config --global protocol.version 2
git checkout -f
git pull
cd /D D:\src_builds\llvm-monorepo\llvm-build-vs2019

To build:

cmake -Thost=x64 -DLLVM_TARGETS_TO_BUILD:STRING="X86" -DCLANG_PLUGIN_SUPPORT="OFF" -DLLVM_INCLUDE_DOCS:BOOL="OFF" -DLLVM_INCLUDE_EXAMPLES:BOOL="OFF" -DLLVM_INCLUDE_TESTS:BOOL="OFF" -DLLVM_INCLUDE_BENCHMARKS:BOOL="OFF" -DLLVM_ENABLE_PROJECTS:STRING="clang;lld;polly;compiler-rt" -DBUILD_SHARED_LIBS:BOOL="OFF" -DLLVM_ENABLE_ASSERTIONS:BOOL="OFF" -DLLVM_USE_CRT_RELEASE:STRING="MT" -DCMAKE_C_FLAGS_RELEASE:STRING="/MT /O2 /GS- /Gw /DNDEBUG /Qpar /arch:AVX" -DCMAKE_CXX_FLAGS_RELEASE:STRING="/MT /O2 /GS- /Gw /DNDEBUG /Qpar /arch:AVX" -DCMAKE_EXE_LINKER_FLAGS_RELEASE:STRING="/INCREMENTAL:NO /FIXED:NO /OPT:REF /OPT:ICF /CGTHREADS:8" -DCMAKE_SHARED_LINKER_FLAGS_RELEASE:STRING="/INCREMENTAL:NO /FIXED:NO /OPT:REF /OPT:ICF /CGTHREADS:8" -G "Visual Studio 15 2017 Win64" ..\llvm\llvm
cmake --build . --config Release --target ALL_BUILD

Alternatively, to customize build for parallel building:

cmake -Thost=x64 -DLLVM_TARGETS_TO_BUILD:STRING="X86" -DCLANG_PLUGIN_SUPPORT="OFF" -DLLVM_INCLUDE_DOCS:BOOL="OFF" -DLLVM_INCLUDE_EXAMPLES:BOOL="OFF" -DLLVM_INCLUDE_TESTS:BOOL="OFF" -DLLVM_INCLUDE_BENCHMARKS:BOOL="OFF" -DLLVM_ENABLE_PROJECTS:STRING="clang;lld;polly;compiler-rt" -DBUILD_SHARED_LIBS:BOOL="OFF" -DLLVM_ENABLE_ASSERTIONS:BOOL="OFF" -DLLVM_USE_CRT_RELEASE:STRING="MT" -DCMAKE_C_FLAGS_RELEASE:STRING="/MT /O2 /GS- /Gw /DNDEBUG /cgthreads8 /MP32 /Qpar /arch:AVX" -DCMAKE_CXX_FLAGS_RELEASE:STRING="/MT /O2 /GS- /Gw /DNDEBUG /cgthreads8 /MP32 /Qpar /arch:AVX" -DCMAKE_EXE_LINKER_FLAGS_RELEASE:STRING="/INCREMENTAL:NO /FIXED:NO /OPT:REF /OPT:ICF /CGTHREADS:8" -DCMAKE_SHARED_LINKER_FLAGS_RELEASE:STRING="/INCREMENTAL:NO /FIXED:NO /OPT:REF /OPT:ICF /CGTHREADS:8" -G "Visual Studio 15 2017 Win64" ..\llvm\llvm
cmake --build . --config Release --target ALL_BUILD -- /maxcpucount:8

Adjust /cgthreads, /MP, and /maxcpucount according to local system specs, utilization, etc. YMMV.