[source maps] fix source mapping when there are multiple matching rules

D104406 introduced an error in which, if there are multiple matchings rules for a given path, lldb was only checking for the validity in the filesystem of the first match instead of looking exhaustively one by one until a valid file is found.

Besides that, a call to consume_front was being done incorrectly, as it was modifying the input, which renders subsequent matches incorrect.

I added a test that checks for both cases.

Differential Revision: https://reviews.llvm.org/D106723