Use LLDB_SRC for relative paths

Summary:
Going from LLDB_SRC instead of the file path is safer when looking for compiler-rt. Also need to add support for looking inside the LLVM runtimes subdirectory.

Eventually we should just get CMake to provide these paths during configuration.

Reviewers: tfiala, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D25489

llvm-svn: 284045