[PATCH] Options.td: remove some option groups

[Hans Wennborg]

  I've been trying to get my head around the rationale behind the option groups in Options.td. This is how it seems to work currently:

  - Some groups provide HelpText which gets printed by the -help option

  - Some groups provide functionality, i.e. they're actually referenced by code. For example, the driver will determine its action by looking at the last option in Action_Group, decide the opt level by last option in O_Group, etc.

  - Some groups provide functionality by being part of another group. For example, f_Group doesn't provide any functionality in itself, but it's part of CompileOnly_Group which is referenced by code. This means we could s/f_Group/CompileOnly_Group/ without any functionality change.

  - Some groups might have a documentary function. For example, m_hexagon_features_Group doesn't actually do anything, but it tells the reader of Optionts.td that options such as -mieee-rnd-near are Hexagon related.

  This still leaves us with groups with unclear value. For example, the only purpose of m_Group seems to be to contain all options that start with 'm'. Or most of them; e.g. -maltivec isn't part of m_Group, and others might be missing too - we wouldn't notice since the group doesn't have any real function.

  Should we kill the groups that don't have any function or clear documentary purpose (the f_Group and m_Group are the big examples here)?

  I've uploaded a patch that removes the ones that seem to be obviously useless.

http://llvm-reviews.chandlerc.com/D1227