[CMake] Fixing lit for runtimes directory

Copy variable LLVM_BUILD_MAIN_SRC_DIR from LLVMConfig.cmake to
LLVM_MAIN_SRC_DIR as it is named for in-tree builds. This ensures that
add_lit_target() can reliably find llvm-lit which is not necessarily
in the PATH.

Differential Revision: https://reviews.llvm.org/D24503

llvm-svn: 281585