[lld] Convert LLVM_CMAKE_PATH to a CMake path

Otherwise I get the following error on windows.
```
CMake Error at D:/bld/lld_1569206597988/work/build/CMakeFiles/CMakeTmp/CMakeLists.txt:2 (set):
  Syntax error in cmake code at

    D:/bld/lld_1569206597988/work/build/CMakeFiles/CMakeTmp/CMakeLists.txt:2

  when parsing string

    D:\bld\lld_1569206597988\_h_env\Library\lib\cmake\llvm

  Invalid character escape '\b'.

CMake Error at D:/bld/lld_1569206597988/_build_env/Library/share/cmake-3.15/Modules/CheckSymbolExists.cmake:100 (try_compile):
  Failed to configure test project build system.
Call Stack (most recent call first):
  D:/bld/lld_1569206597988/_build_env/Library/share/cmake-3.15/Modules/CheckSymbolExists.cmake:57 (__CHECK_SYMBOL_EXISTS_IMPL)
  D:/bld/lld_1569206597988/_h_env/Library/lib/cmake/llvm/HandleLLVMOptions.cmake:943 (check_symbol_exists)
  CMakeLists.txt:56 (include)
```

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D68158