[Frontend] Recognize environment variable SOURCE_DATE_EPOCH

See https://reproducible-builds.org/docs/source-date-epoch/ . The environment
variable ``SOURCE_DATE_EPOCH`` been recognized by many compilers.

In GCC, if `SOURCE_DATE_EPOCH` is set, it specifies a UNIX timestamp to be used
in replacement of the current date and time in the `__DATE__` and `__TIME__`
macros. Note: GCC as of today does not update `__TIMESTAMP__` (the modification
time of the current source file) but
https://wiki.debian.org/ReproducibleBuilds/TimestampsFromCPPMacros expresses the
intention to update it.

This patches parses SOURCE_DATE_EPOCH and changes all the three macros.

In addition, in case gmtime/localtime returns null (e.g. on 64-bit Windows
gmtime returns null when the timestamp is larger than 32536850399
(3001-01-19T21:59:59Z)), use `??? ?? ????` as used by GCC.

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D135045