Improve natvis for llvm::SmallString so that it correctly displays only the valid...

Improve natvis for llvm::SmallString so that it correctly displays only the valid portion of the string

The usual method, and the one employed before my change, of displaying strings in natvis is to make use of the "<variable>,s" format specifier; however, this method only works for null-terminated strings. My fix here is to use the "<pointer>,[size]" format specifier to display a bounded array, and then cast it to "const char*", which in the MSVC debugger has the desired effect of rendering the character array as a string.

Differential Revision: https://reviews.llvm.org/D27972

llvm-svn: 290224