AsmParser: record "# line file" context to calculate location for diag

Since we can't emit diagnostics for missing "jmp 1f" labels until the end of
the file, we need to be able to restore the context used to calculate
file/line. This is basically the "# line file" directive that's being used at
the time the expression is seen.

rdar://25706972

llvm-svn: 266238