[clang] Move getenv call for SOURCE_DATE_EPOCH out of frontend NFC

Move the check for SOURCE_DATE_EPOCH to the driver and use a cc1 option
to pass it to the frontend. This avoids hidden state in the cc1
invocation and makes this env variable behave more like other env
variables that clang handles in the driver.

Differential Revision: https://reviews.llvm.org/D136717