Fix a subtle issue in SmallVector. The following code did not work as expected:

Fix a subtle issue in SmallVector.  The following code did not work as expected:
  vec.insert(vec.begin(), vec[3]);
The issue was that vec[3] returns a reference into the vector, which is invalidated when insert() memmove's the elements down to make space.  The method needs to specifically detect and handle this case to correctly match std::vector's semantics.

Thanks to Howard Hinnant for clarifying the correct behavior, and explaining how std::vector solves this problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134554 91177308-0d34-0410-b5e6-96231b3b80d8