[llvm] r199818 - Fix inline assembly that switches between ARM and Thumb modes

[Rafael Espíndola]

> The problem we need to solve is code like this:
>
>   int foo(int a, int b) {
>     int r = a + b;
>     asm volatile(
>         ".align 2     \n"
>         ".arm         \n"
>         "add r0,r0,r0 \n"
>     : : "r"(r));
>     return r+1;
>   }
>
> If we compile this function in thumb mode then the inline assembly
> will switch to arm mode. We need to make sure that we switch back to
> thumb mode after emitting the inline assembly or we will incorrectly
> encode the instructions that follow (i.e. the assembly instructions
> for return r+1).

Is this supported by GCC too? If not, should we document it as an extension?

Why is this needed at the MCStreamer level? Why can't codegen use
EmitAssemblerFlag directly?

Cheers,
Rafael