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

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@243950 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang