Pretty Printer: Fix printing of conversion operator decls and calls.

- Don't emit anything when we encounter a call to a conversion operator.
    "bar(a & b)" instead of "bar(a & b.operator int())"
  This preserves the semantics and is still idempotent if we print the AST multiple times.

- Properly print declarations of conversion operators.
    "explicit operator bool();" instead of "bool operator _Bool();"

PR18776.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202167 91177308-0d34-0410-b5e6-96231b3b80d8