[compiler-rt][lit] initialize LIT LLVMConfig instance

to handle lit tools searching. Otherwise
compiler-rt depends on system environment variable PATH for lit tools
which diverge from the other LLVM projects. This reverts D83486 which
really should be implemented in LIT itself when the PATH is constructed.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D122837