[test] Fix finding LLDB tools when building stand-alone

Use both LLDB- and LLVM-specific tool/library directories when LLDB is
being built stand-alone. This ensures that the freshly-built tools
(and libraries) are used correctly.

Without this patch, the test suite uses LLVM_TOOLS_DIR and LLVM_LIBS_DIR
to locate lldb, and set PATH and LD_LIBRARY_PATH. When doing
a stand-alone build, these variables represent the installed LLVM.
As a result, tests either fail due to missing lldb executable
or use an earlier installed LLDB version rather than the one being
built.

To solve this, additional LLDB_TOOLS_DIR and LLDB_LIBS_DIR variables
are added and populated using LLVM_*_OUTPUT_INTDIR. Those variables
contain directories used to output built executables and libraries.
In stand-alone builds, they represent the build-tree directories
used by LLDB. In integrated builds, they have the same values as
LLVM_*_DIR and therefore using them does not harm.

The new variables are prepended to PATH and LD_LIBRARY_PATH to ensure
that freshly built binaries are preferred over potentially earlier
installed ones. Furthermore, paths used to locate various tools are
updated to match appropriate locations.

Differential Revision: https://reviews.llvm.org/D29985

llvm-svn: 295621