[ARM] Remove FeatureNoARM implies ModeThumb.

Summary:
By removing FeatureNoARM implies ModeThumb, we can detect cases where a
function's target-features contain -thumb-mode (enables ARM codegen for the
function), but the architecture does not support ARM mode. Previously, the
implication caused the FeatureNoARM bit to be cleared for functions with
-thumb-mode, making the assertion in ARMSubtarget::ARMSubtarget [1]
pointless for such functions.

This assertion is the only guard against generating ARM code for
architectures without ARM codegen support. Is there a place where we
could easily generate error messages for the user? At the moment, we
would generate ARM code for Thumb-only architectures. X86 has the same
behavior as ARM, as in it only has an assertion and no error message,
but I think for ARM an error message would be helpful. What do you
think?

For the example below, `llc -mtriple=armv7m-eabi test.ll -o -` will
generate ARM assembler (or fail with an assertion error with this patch).
Note that if we run the resulting assembler through llvm-mc, we get
an appropriate error message, but not when codegen is handled
through clang.

```
define void @bar() #0 {
entry:
  ret void
}

attributes #0 = { "target-features"="-thumb-mode" }
```

[1] https://github.com/llvm-mirror/llvm/blob/c1f7b54cef62e9c8aa745d40bea146a167bf844e/lib/Target/ARM/ARMSubtarget.cpp#L147

Reviewers: t.p.northover, rengolin, peter.smith, aadg, silviu.baranga, richard.barton.arm, echristo

Reviewed By: rengolin, echristo

Subscribers: efriedma, aemerson, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D35569

llvm-svn: 310476