[libc] Add implementation of difftime function.

[rtenneti]

Summary

The difftime function computes the difference between two calendar
times: time1 - time0 as per as per 7.27.2.2 section in
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2478.pdf.

double difftime(time_t time1, time_t time0);

Tested:
Unit tests

Co-authored-by: Jeff Bailey <[e-mail]>

[jeffbailey]

Can you please update the status doc at
https://libc.llvm.org/date_and_time.html ?

• smime.p7s3 KBDownload

[rtenneti]

Updated doc and fixed formatting issues.

[rtenneti]

Updated the Doc. Thanks Jeff. PTAL.

libc/docs/date_and_time.rst

 clock
 clock_getcpuclockid
 clock_getres
 clock_gettime         |check|
 clock_nanosleep
 clock_settime
 ctime
 ctime_r
-difftime
+difftime              |check|
 getdate
 gmtime                |check|
 gmtime_r              |check|
 localtime
 localtime_r
 mktime                |check|
 nanosleep             |check|
 strftime

libc/src/time/difftime.h

-//===-- Implementation header of difftime -------------------------*- C++
-//-*-===//
+//===-- Implementation header of difftime -----------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_TIME_DIFFTIME_H

libc/src/time/difftime.cpp

-//===-- Implementation of difftime function
-//---------------------------------===//
+//===-- Implementation of difftime function -------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/time/difftime.h"