[flang] COMMAND_ARGUMENT_COUNT runtime implementation

Grab whatever ProgramStart has stored in executionEnvironment.argc and
subtract 1 (based on the assumption that ProgramStart is called with
a C-style argc that counts the command name as an argument).

Spoiler alert: The tests will evolve into fixtures when we implement
GET_COMMAND_ARGUMENT etc.

Differential Revision: https://reviews.llvm.org/D109048