[lldb] Fix Python interpreter workaround (attempt #2)

On macOS, to make DYLD_INSERT_LIBRARIES and the Python shim work
together, we have a workaroud that copies the "real" Python interpreter
into the build directory. This doesn't work when running in a virtual
environment, as the copied interpreter cannot find the packages
installed in the virtual environment relative to itself.

Address this issue by copying the Python interpreter into the virtual
environment's `bin` folder, rather than the build folder, when the test
suite detects that it's being run inside a virtual environment.

I'm not thrilled about this solution because it puts a file outside the
build directory. However, given virtual environments are considered
disposable, this seems reasonable.

lldb/test/API/lit.cfg.py

@@ -63,13 +63,14 @@ def find_python_interpreter():
     if "DYLD_INSERT_LIBRARIES" not in config.environment:
         return None
 
-    # If we're running in a virtual environment, we already have a copy of the
-    # Python executable.
+    # If we're running in a virtual environment, we have to copy Python into
+    # the virtual environment for it to work.
     if sys.prefix != sys.base_prefix:
-        return None
+        copied_python = os.path.join(sys.prefix, "bin", "copied-python")
+    else:
+        copied_python = os.path.join(config.lldb_build_directory, "copied-python")
 
     # Avoid doing any work if we already copied the binary.
-    copied_python = os.path.join(config.lldb_build_directory, "copied-python")
     if os.path.isfile(copied_python):
         return copied_python