Create a new PrintingPolicy class, which we pass down through the AST

Create a new PrintingPolicy class, which we pass down through the AST
printing logic to help customize the output. For now, we use this
rather than a special flag to suppress the "struct" when printing
"struct X" and to print the Boolean type as "bool" in C++ but "_Bool"
in C.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72590 91177308-0d34-0410-b5e6-96231b3b80d8