[CMake] Include CheckSymbolExists in HandleLLVMOptions

[phosek]

Summary

In LLVM build, HandLLVMOptions relies on CheckSymbolExists being
included by config-ix, but HandleLLVMOptions is also being imported
from other parts of the build such as the runtimes build, libunwind,
libcxxabi, libcxx, and none of those include LLVM's config-ix which
is breaking the build. We should include CheckSymbolExists directly
in HandleLLVMOptions to make it self-contained rather than relying
on config-ix.

[phosek]

@dsanders already made this change in rL354389.

llvm/cmake/modules/HandleLLVMOptions.cmake

 # This CMake module is responsible for interpreting the user defined LLVM_
 # options and executing the appropriate CMake commands to realize the users'
 # selections.
 
 # This is commonly needed so make sure it's defined before we include anything
 # else.
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 include(CheckCompilerVersion)
 include(HandleLLVMStdlib)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
+include(CheckSymbolExists)
 
 if(CMAKE_LINKER MATCHES "lld-link\.exe" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld") OR LLVM_ENABLE_LLD)
   set(LINKER_IS_LLD_LINK TRUE)
 else()
   set(LINKER_IS_LLD_LINK FALSE)
 endif()
 
 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")