[lldb-vscode] Fix coredump load source mapping for first file

SetSourceMapFromArguments is called after the core is loaded. This means
that the source file for the crashing code won't have the source map applied.
Move the call to SetSourceMapFromArguments in request_attach to just after
the call to RunInitCommands, matching request_launch behavior.

Reviewed By: clayborg, wallace

Differential Revision: https://reviews.llvm.org/D112834