clang: C++1y deprecated attribute message

[Joseph Mansfield]

The attached patch adds support for deprecation messages in C++1y. This is
my first time hacking with clang so a code review would definitely be
needed.

As an example of what is now supported:

[[deprecated("use bar instead")]] void foo();

When this function is used, the following warning is emitted:

warning: 'foo' is deprecated: use bar instead [-Wdeprecated-declarations]
  foo();
  ^

Some tests included.

Known potential issues:
- Attribute parsing currently doesn't seem to distinguish between C++11 and
C++1y attributes.
- Maybe errors like [[deprecated("foo", "bar")]] would better report "too
many arguments" than "expected ')'". Not sure about the best way to go
about this.
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