[CMake] Don't cache LLVM_MAIN_SRC_DIR

[phosek]

Summary

If we're not in a standalone build, this variable should be already
set, so there's no need to set it again or to cache it.

[smeenai]

@mgorny mentioned a need for this to be cached in D57406#1376170, so I'll let him weigh in on this.

[mgorny]

I suppose you are correct. It shouldn't need to be redefined when doing in-tree build.

Disclaimer: I haven't tested it. Presuming you've tested both variants.

libunwind/CMakeLists.txt

if (LLVM_LIT)
     endif()
   else()
     set(LLVM_INCLUDE_TESTS OFF)
   endif()
 
   set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
   set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
 else()
-  set(LLVM_MAIN_SRC_DIR "${CMAKE_SOURCE_DIR}" CACHE PATH "Path to LLVM source tree")
   set(LLVM_LIT "${CMAKE_SOURCE_DIR}/utils/lit/lit.py")
 endif()
 
 #===============================================================================
 # Setup CMake Options
 #===============================================================================
 include(CMakeDependentOption)
 include(HandleCompilerRT)