Fix handling of the arm IT instruction in the unwinder

The IT instruction can specify condition code for up to 4 consecutive
instruction and it is used quite often by clang in epilogues causing
an issue when trying to unwind from locations covered by the IT
instruction and for locatins inmediately after the IT instruction.

Changes made to fix it:
* Introduce the concept of conditional instruction block what is a list
  of consecutive instructions with the same condition. We update the
  unwind information during the conditional instruction block and when
  we reach the end of it (first instruction with a differemt condition)
  then we restore the unwind information we had before the condition.
* Fix a bug in the ARM instruction emulator where neither PC nor the
  ITSTATE was advanced when we reached an instruction what we can't
  decode.

After the change we have no regression on android-arm running the
regular test suit and TestStandardUnwind also passes when running it
with clang as the compiler (previously it failed on an IT instruction).

Differential revision: http://reviews.llvm.org/D16814

llvm-svn: 260368

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