[llvm-dev] Cannot build Clang/LLVM on Windows with LLVM_BUILD_LLVM_DYLIB

[biologi spm via llvm-dev]

I am trying to build Clang/LLVM with -DLLVM_BUILD_LLVM_DYLIB=ON and
-DLLVM_LINK_LLVM_DYLIB=ON to reduce the size of toolchain.

I am getting LINK error 1181 about `lib\LLVM.lib` not found. After
investigating, `lib\LLVM.lib` is supposed to be produced along with
`bin\LLVM.dll`. `bin\LLVM.dll` is indeed produced, but it does not
contain any exported symbols (10KB only) because LLVM does not use
`__declspec(dllexport)` and `__declspec(dllimport)`, so lib.exe
refuses to produce `lib\LLVM.lib`.

If I just build static-linked Clang/LLVM (default), it works fine.

Windows bot of WebAssembly waterfall is also affected:
https://github.com/WebAssembly/waterfall/commit/2051fd4cdb2b4017e66dfcd70203d40f3d0b461d

Compiler: Visual Studio 2017

CMake configuration:

cmake -GNinja -Bbuild -Hllvm -DCMAKE_BUILD_TYPE=Release
-DLLVM_INCLUDE_EXAMPLES=OFF
-DLLVM_INCLUDE_TESTS=OFF
-DCMAKE_INSTALL_PREFIX=d:\a\1\b
-DLLVM_TARGETS_TO_BUILD=X86
-DLLVM_INSTALL_TOOLCHAIN_ONLY=ON
-DLLVM_BUILD_LLVM_DYLIB=ON
-DLLVM_LINK_LLVM_DYLIB=ON
-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly