Clang: Look for llvm-lib when using MSVC-like front-end

In commit 55196a1440 (MSVC: Use 'lib' instead of 'link /lib' to create
static libraries, 2020-01-10, v3.18.0-rc1~625^2) we changed CMake to use
lib instead of `link /lib` to create static libraries, but it didn't
search for `llvm-lib`. If you have `llvm-lib` but not `lib` (e.g. when
cross-compiling), when `CMakeFindBinutils` is invoked for the `C` and
`CXX` languages, `CMAKE_AR` is not found. When it's subsequently invoked
for the ASM language, `CMAKE_ASM_SIMULATE_ID` and
`CMAKE_ASM_COMPILER_FRONTEND_VARIANT` are not set (because
`CMakeDetermineASMCompiler` doesn't call `CMAKE_DETERMINE_COMPILER_ID`,
which sets those variables), so we go down the non-MSVC conditional and
set `CMAKE_AR` to a GNU-style `ar`, which of course does not understand
lib flags. Explicitly search for `llvm-lib` to avoid this situation.

Modules/CMakeFindBinUtils.cmake

@@ -72,10 +72,11 @@ if(("x${CMAKE_${_CMAKE_PROCESSING_LANGUAGE}_SIMULATE_ID}" STREQUAL "xMSVC" AND
   if("x${CMAKE_${_CMAKE_PROCESSING_LANGUAGE}_COMPILER_ID}" STREQUAL "xClang")
     find_program(CMAKE_NM NAMES ${_CMAKE_TOOLCHAIN_PREFIX}nm llvm-nm HINTS ${_CMAKE_TOOLCHAIN_LOCATION})
     set(_CMAKE_ADDITIONAL_LINKER_NAMES "lld-link")
+    set(_CMAKE_ADDITIONAL_AR_NAMES "llvm-lib")
   endif()
 
   find_program(CMAKE_LINKER NAMES ${_CMAKE_ADDITIONAL_LINKER_NAMES} link HINTS ${_CMAKE_TOOLCHAIN_LOCATION})
-  find_program(CMAKE_AR     NAMES lib  HINTS ${_CMAKE_TOOLCHAIN_LOCATION})
+  find_program(CMAKE_AR     NAMES ${_CMAKE_ADDITIONAL_AR_NAMES}     lib  HINTS ${_CMAKE_TOOLCHAIN_LOCATION})
   find_program(CMAKE_MT     NAMES mt   HINTS ${_CMAKE_TOOLCHAIN_LOCATION})
 
   list(APPEND _CMAKE_TOOL_VARS LINKER MT)