[lldb-vscode] correctly handle multiple sourceMap entries

Summary:
`lldb-vscode` concatenates a string of sourceMap entries
specified in the config, but fails to put a space between
each entry, which causes the settings command to fail.
This patch adds a space between each mapping.

Patch by Richard Howell

Differential Revision: https://reviews.llvm.org/D67569

llvm-svn: 373016