[source map] fix relative path breakpoints

[source map] fix relative path breakpoints

https://reviews.llvm.org/D45592 added a nice feature to be able to specify a breakpoint by a relative path. E.g. passing foo.cpp or bar/foo.cpp or zaz/bar/foo.cpp is fine. However, https://reviews.llvm.org/D68671 by mistake disabled the test that ensured this functionality works. With time, someone made a small mistake and fully broke the functionality.

So, I'm making a very simple fix and the test passes.

Differential Revision: https://reviews.llvm.org/D107126