When lldb stops in a stack frame where we have source level information (file, line number), don't

print the disassembly context around $pc -- just print the filename and line number, even if we 
can't show the source code.  Previously if the source file was not available, lldb would print
the source filename & line number and assembly.  
<rdar://problem/13072951> 

llvm-svn: 180706