llvm-vtabledump: use a std::map instead of a StringMap for VBTables

StringMap doesn't guarantee any particular iteration order,
this is suboptimal when comparing llvm-vtabledump's output for two
object files.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213921 91177308-0d34-0410-b5e6-96231b3b80d8