[cmake] Fix clang-cl cross-compilation on macOS

[smeenai]

Summary

macOS paths usually start with /Users, which clang-cl interprets as a
macro undefine, leading to pretty much everything failing to compile.

CMake should be taught to put a -- in its compilation rules for clang-cl
(and I've been meaning to submit that upstream for a while). In the
meantime, however, and to support older CMake versions, we can just
create a custom make rules override to fix the compilation rules.

[compnerd]

This is awesome!

llvm/trunk/cmake/platforms/ClangClCMakeCompileRules.cmake

+# macOS paths usually start with /Users/*. Unfortunately, clang-cl interprets
+# paths starting with /U as macro undefines, so we need to put a -- before the
+# input file path to force it to be treated as a path. CMake's compilation rules
+# should be tweaked accordingly, but until that's done, and to support older
+# CMake versions, overriding compilation rules works well enough. This file will
+# be included by cmake after the default compilation rules have already been set
+# up, so we can just modify them instead of duplicating them entirely.
+string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_C_COMPILE_OBJECT "${CMAKE_C_COMPILE_OBJECT}")
+string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_CXX_COMPILE_OBJECT "${CMAKE_CXX_COMPILE_OBJECT}")

llvm/trunk/cmake/platforms/WinMsvc.cmake

 set(_CMAKE_SHARED_LINKER_FLAGS_INITIAL "${CMAKE_SHARED_LINKER_FLAGS}" CACHE STRING "")
 set(CMAKE_SHARED_LINKER_FLAGS "${_CMAKE_SHARED_LINKER_FLAGS_INITIAL} ${LINK_FLAGS}" CACHE STRING "" FORCE)
 
 # CMake populates these with a bunch of unnecessary libraries, which requires
 # extra case-correcting symlinks and what not. Instead, let projects explicitly
 # control which libraries they require.
 set(CMAKE_C_STANDARD_LIBRARIES "" CACHE STRING "" FORCE)
 set(CMAKE_CXX_STANDARD_LIBRARIES "" CACHE STRING "" FORCE)
+
+# Allow clang-cl to work with macOS paths.
+set(CMAKE_USER_MAKE_RULES_OVERRIDE "${CMAKE_CURRENT_LIST_DIR}/ClangClCMakeCompileRules.cmake")