GettingStarted.md needs some tweaks for Linux

[grynspan]

The following issues should be addressed in GettingStarted.md when you're trying to build for Linux:

1. lldb is not built by default, so folks should pass -l to build-script to ensure it's built;
2. $PATH needs to have the build results of llvm-project and swift added to it (possibly others); and
3. $LLVM_SYMBOLIZER_PATH should be set.

(Will update if I find more issues before I get to a PR.)