[MC][DWARF] Corrected handling of is_stmt flag in .loc directives

According to DWARF standard, is_stmt is a global flag; when set or cleared it should affect subsequent .loc directives.

However llvm assembler handled is_stmt differently: it forced all locations to have is_stmt=1 unless is_stmt was specified explicitly as 0.

The fix utilizes current DWARF state flags to compute correct is_stmt values.

See https://bugs.llvm.org/show_bug.cgi?id=45529 for a detailed issue description.

Reviewers: arsenm, probinson, enderby

Differential Revision: https://reviews.llvm.org/D78102