Make the tautological out of range warning use Sema::DiagRuntimeBehavior so that
the warning will not trigger on code protected by compile time checks.
ARM IAS: support implicit immediate 0s for {LD,ST}R{B,}T

The ARM ARM indicates the mnemonics as follows:

  ldrbt{<c>}{<q>} <Rt>, [<Rn>], {, #+/-<imm>}
  ldrt{<c>}{<q>} <Rt>, [<Rn>] {, #+/-<imm>}
  strbt{<c>}{<q>} <Rt>, [<Rn>] {, #<imm>}
  strt{<c>}{<q>} <Rt>, [<Rn>] {, #+/-<imm>}

This improves the parser to deal with the implicit immediate 0 for the mnemonics
as per the specification.

Thanks to Joerg Sonnenberger for the tests!
ARM IAS: support GNU extension for ldrd, strd

The GNU assembler has an extension that allows for the elision of the paired
register (dt2) for the LDRD and STRD mnemonics.  Add support for this in the
assembly parser.  Canonicalise the usage during the instruction parsing from
the specified version.
ARM IAS: support #:{lower,upper}16: for GNU compatibility

The GNU assembler supports prefixing the expression with a '#' to indiciate that
the value that is being moved is infact a constant.  This improves the
compatibility of the integrated assembler's parser for this.