[lldb] Get rid of LLDB_LIB_DIR and LLDB_IMPLIB_DIR in dotest

This patch removes the rather confusing LLDB_LIB_DIR and LLDB_IMPLIB_DIR
environment variables. They are confusing because LLDB_LIB_DIR would
point to the bin subdirectory in the build root while LLDB_IMPLIB_DIR
would point to the lib subdirectory. The reason far this was
LLDB.framework, which gets build under bin.

This patch replaces their uses with configuration.lldb_framework_path
and configuration.lldb_libs_dir respectively.

Differential revision: https://reviews.llvm.org/D86817