[NFC][RemoveDIs] Shuffle header inclusions for "new" debug-info

BasicBlock.h and Instruction.h will eventually need to include
DebugProgramInstruction.h so that debug-info attached to instructions can
be enumerated and cloned. Originally including it made compiling clang
much slower, I think I've pinned that down as being the inclusion of
DebugInfoMetadata.h causing ~every LLVM translation unit to parse
all the debug-info classes.

This patch avoids that by shifting some functions into the cpp file rather
than the header, and restores the inclusion of DebugProgramInstruction.h in
BasicBlock.h so that the rest of the RemoveDIs functionality can land.