Merging r243945-243950.

Merging r243945-243950.
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r243945 | chandlerc | 2015-08-03 20:52:52 -0700 (Mon, 03 Aug 2015) | 5 lines

[UB] Fix two cases of UB in copy/pasted code from SmallVector.

We should really stop copying and pasting code around. =/

Found by UBSan.
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r243946 | chandlerc | 2015-08-03 20:52:56 -0700 (Mon, 03 Aug 2015) | 9 lines

[UB] Fix the two ways that we would try to memcpy from a null buffer in
the nested name specifier code.

First, skip the entire thing when the input is empty.

Next, handle the case where we started off with a null buffer and a zero
capacity to skip copying and freeing.

This was found with UBSan.
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r243947 | chandlerc | 2015-08-03 20:52:58 -0700 (Mon, 03 Aug 2015) | 4 lines

[UB] When attaching empty strings to the AST, use an empty StringRef
rather than forcing the bump pointer allocator to produce a viable
pointer. This also fixes UB when we would try to memcpy from the null
incoming StringRef.
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r243948 | chandlerc | 2015-08-03 20:53:00 -0700 (Mon, 03 Aug 2015) | 9 lines

[UB] Another place where we were trying to put string data into
a BumpPtrAllocator. This at least now handles the case where there is no
concatentation without calling memcpy on a null pointer. It might be
interesting to handle the case where everything is empty without
round-tripping through the allocator, but it wasn't clear to me if the
pointer returned is significant in any way, so I've left it in
a conservatively more-correct state.

Again, found with UBSan.
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r243949 | chandlerc | 2015-08-03 20:53:01 -0700 (Mon, 03 Aug 2015) | 4 lines

[UB] Guard two calls to memcpy in generated attribute code to handle
null StringRef objects as inputs.

Found by UBSan.
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r243950 | chandlerc | 2015-08-03 20:53:04 -0700 (Mon, 03 Aug 2015) | 8 lines

[UB] Avoid a really broken call to realloc that would later result in
a bad call to memcpy.

When we only have a buffer from one of the two reparse calls, we can
just return that buffer rather than going through the realloc/memcpy
dance.

Found with UBsan.
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_37@244223 91177308-0d34-0410-b5e6-96231b3b80d8