Add option -polly-codegen-add-debug-printing

When this option is enabled, Polly will emit printf calls for each scalar
load/and store which dump the scalar value loaded/stored at run time.

This patch also refactors the RuntimeDebugBuilder to use variadic templates
when generating CPU printfs. As result, it now becomes easier to print
strings that consist of a set of arguments. Also, as a single printf
call is emitted, it is more likely for such strings to be emitted atomically
if executed multi-threaded.

llvm-svn: 246941