[PATCH] Fix for assertion when compiling for Thumb1, with a processor with a VFP unit

[Oliver Stannard]

> Does this require a corresponding Clang change?

Clang already does not seem to take into account whether a VFP unit is present, so I don't think that this patch requires a clang change.

However, a bit of experimentation shows that it is possible to make things go very wrong by specifying a hard-float calling convention with an architecture which does not support it. The problems are not limited to clang making assumptions about llvm's default calling convention. There should probably be some logic in clang to make sure the user is not trying to do something architecturally impossible, but that is a bigger issue than this patch.

> Wouldn't it be simpler (and more explicit) to early return?

There is still one case in which we return CallingConv::ARM_**A**PCS rather than CallingConv::ARM_**AA**PCS, so I don't think this would simplify things.

http://reviews.llvm.org/D4030