This lets lldb give sane output for SmallVectors, e.g.
Before:
(lldb) p sv
(llvm::SmallVector<int, 10>) $0 = {
  (llvm::SmallVectorImpl<int>) llvm::SmallVectorImpl<int> = {
    (llvm::SmallVectorTemplateBase<int>) llvm::SmallVectorTemplateBase<int> = {
      (llvm::SmallVectorTemplateCommon<int>) llvm::SmallVectorTemplateCommon<int> = {
        (llvm::SmallVectorBase) llvm::SmallVectorBase = {
          (void *) BeginX = 0x00007fff5fbff960
...
}

After:
(lldb) p sv
(llvm::SmallVector<int, 10>) $0 = {
  (int) [0] = 42
  (int) [1] = 23
...
}

The script is still a bit rough so expect crashes for vectors of complex types.
Synthetic children are _not_ available in xcode 4.2, newer LLDBs should work though.

llvm-svn: 148308