[Sanitizer] Introduce "stack_trace_format" runtime flag.

This flag can be used to specify the format of stack frames - user
can now provide a string with placeholders, which should be printed
for each stack frame with placeholders replaced with actual data.
For example "%p" will be replaced by PC, "%s" will be replaced by
the source file name etc.

"DEFAULT" value enforces default stack trace format currently used in
all the sanitizers except TSan.

This change also implements __sanitizer_print_stack_trace interface
function in TSan.

llvm-svn: 221469