[PATCH] Forward -target-feature option through to assembler.

[Charlie Turner]

The `cc1as` driver supports the `-target-feature` option, but the Clang driver refuses to forward such an option specified with `-Xassembler` or `-Wa` through to `cc1as`, claiming it is unrecognized.  This patch makes Clang forward the `target-feature` option and its argument through to the assembler.

The `-target-feature` option is useful for enabling/disabling optional features when using Clang as an assembler. For example (assuming `test.s` contains appropriate content),

```
$ clang -c -target arm-none-eabi -mcpu=cortex-a9 -Xassembler -target-feature -Xassembler -mp test.s
# Or
$ clang -c -target arm-none-eabi -mcpu=cortex-a9 -Wa,-target-feature,-mp test.s
```

Would signal an error if the assembly file `test.s` contained any multiprocessing extension instructions, such as `PLDW`,

```
$ clang -c -target arm-none-eabi -mcpu=cortex-a9 -Xassembler -target-feature -Xassembler -mp test.s
test.s:31:2: error: instruction requires: mp-extensions
        pldw    [r0]
```

However, currently the behavior is,

```
$ clang -c -target arm-none-eabi -mcpu=cortex-a9 -Xassembler -target-feature -Xassembler -mp test.s
clang-3.6: error: unsupported argument '-target-feature' to option 'Xassembler'
clang-3.6: error: unsupported argument '-mp' to option 'Xassembler
```

Which I consider incorrect.

http://reviews.llvm.org/D6170

Files:
  lib/Driver/Tools.cpp
  test/Driver/integrated-as.s
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