Parse: return true from ParseCXX11AttributeArgs if an attribute was added

In the case that we diagnosed an invalid attribute due to missing or present
arguments, we would return false, indicating to the caller that the parsing
failed.  However, we would have added the attribute in ParseAttributeArgsCommon
(which may have been called indirectly through ParseGNUAttributeArgs).
Returning true in this case ensures that a second copy of the attribute is not
added.

I haven't added a test case for this as the existing test will cover this with
the next commit which diagnoses a C++14 attribute applied in C++11 mode.  Rather
than duplicating the existing test case, allow the tree to remain without a test
between this and the next change.  We would see double warnings in the
[[deprecated()]] applied to a declaration in C++11 mode, which will cause an
error in the cxx0x-attributes test.

llvm-svn: 229446