Changes to mktime to handle invalid dates, overflow and underflow andcalculating...

Changes to mktime to handle invalid dates, overflow and underflow andcalculating the correct date and thenumber of seconds even if invalid datesare passed as arguments.

Added tests for invalid dates like the following
  Date 1970-01-01 00:00:-1 is treated as 1969-12-31 23:59:59 and seconds
  are returned for the modified date.

Tested the code by doing ninja check-libc (and cmake).

Reviewed By: sivachandra, rtenneti

Differential Revision: https://reviews.llvm.org/D96684