rename Program -> MainProgram to match the spec.
rename CodeGenFunction to CodeGenSubprogram
remove altivec driver test... not currently useful b/c of the C header include
Rename Function to Subprogram

In Fortran, a function is only a particular kind of subprogram. However,
what we'd call a function in C/C++ can be any kind of subprogram in Fortran
(even the main program, which is not really a kind of subprogram, but acts
like one in almost every way).
Change Fn -> SubPgm in many places
PREDEF_TYPE_BUILTIN_FN -> PREDEF_TYPE_BUILTIN_SUBPGM
FUNCTION -> SUBPROGRAM in AST
Revert Function -> SubProgram change for PredefinedExpr