[sourcekit] Fix for Ubuntu 16.04 Swift ASAN bot failures

Don't mix and match between the just-built clang and the host clang when
ASAN is enabled.

rdar://problem/45273748

Author: Nathan Hawes

tools/SourceKit/CMakeLists.txt

@@ -101,7 +101,8 @@ if(NOT "${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
 
   if(SWIFT_BUILD_SOURCEKIT)
     if(CMAKE_C_COMPILER_ID STREQUAL Clang AND
-       CMAKE_C_COMPILER_VERSION VERSION_GREATER 3.8)
+       CMAKE_C_COMPILER_VERSION VERSION_GREATER 3.8
+       OR LLVM_USE_SANITIZER)
      set(SWIFT_SOURCEKIT_C_COMPILER ${CMAKE_C_COMPILER})
      set(SWIFT_SOURCEKIT_CXX_COMPILER ${CMAKE_CXX_COMPILER})
    elseif(${CMAKE_SYSTEM_NAME} STREQUAL ${CMAKE_HOST_SYSTEM_NAME})