This depends on the libc implementation. Use the string from the assertion message instead. Overly specific, but so is this entire test.
Put the guts of `ComputeLineNumbers` into `LineOffsetMapping::get` and `LineOffsetMapping::LineOffsetMapping`. As a drive-by, store the number of lines directly in the bump-ptr-allocated array. Differential Revision: https://reviews.llvm.org/D89913