libcpp: decode original directory strings for traditional CPP

libcpp: decode original directory strings for traditional CPP

In traditional CPP mode (-save-temps, -no-integrated-cpp, etc.), the
compilation directory is conveyed to cc1 using a line such as:

 # <line> "/path/name//"

This string literal can contain escape sequences, for instance, if the
original source file was compiled in "/tmp/a\b", then this line will be:

 # <line> "/tmp/a\\b//"

So reading the compilation directory must decode escape sequences. This
last part is currently missing and this patch implements it.

libcpp/
	* init.cc (read_original_directory): Attempt to decode escape
	sequences with cpp_interpret_string_notranslate.