loop-convert, a C++11 for loop modernizer

A new Clang-based tool which converts for loops to use the range-based
syntax new to C++11. Three kinds of loops can be converted:
 - Loops over statically allocated arrays
 - Loops over containers, using iterators
 - Loops over array-like containers, using operator[] and at()

Each transformation is assigned a confidence level by the tool. The
minimum require confidence level to actually apply the transformation
can be specified on the command line, but the default level should be
fine for most code.

Like other tools based on RefactoringTool, it is easiest to use this
tool with a compilation database.

llvm-svn: 162627