Normalize the LLVM cmake path before appending it to the module path

[hughbe]

Summary

LLVM_CMAKE_PATH has backlashes in it. CMake then tries to append this to CMAKE_MODULE_PATH but gets confused and errors out as it thinks we're providing an escape sequence (that's unknown, causing generation to fail)

E.g "C:\Users\hugh..."

I've been porting swift to Windows/MSVC. This was causing the Windows build to fail and I submitted a PR (https://github.com/apple/swift-lldb/pull/156). Maybe it belongs upstream, however

[hughbe]

Fix preview

[hughbe]

Address feedback from Zachary Turner

[hughbe]

As per discussion on lists

lldb/trunk/cmake/modules/LLDBStandalone.cmake

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
   set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
 
   find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
     NO_DEFAULT_PATH)
 
   set(LLVMCONFIG_FILE "${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
   if(EXISTS ${LLVMCONFIG_FILE})
+    file(TO_CMAKE_PATH "${LLVM_CMAKE_PATH}" LLVM_CMAKE_PATH)
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
     include(${LLVMCONFIG_FILE})
   else()
     message(FATAL_ERROR "Not found: ${LLVMCONFIG_FILE}")
   endif()
 
   # They are used as destination of target generators.
   set(LLVM_RUNTIME_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin)