Remove the AST printer (-ast-print-xml), which is too incomplete and

Remove the AST printer (-ast-print-xml), which is too incomplete and
too low-level to actually be useful but is just interesting enough for
people to try to use it (which won't actually work beyond toy examples).

To bring back the AST printer, it needs to be:
  - Complete, covering all of C/C++/Objective-C
  - Documented, with appropriate Schema against which we can validate
  the output
  - Designed for C/C++/Objective-C, not Clang's specific ASTs
  - Stable across Clang versions
  - Well-tested

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127141 91177308-0d34-0410-b5e6-96231b3b80d8