[AST] String literal operator templates have two template args, not one

[AST] String literal operator templates have two template args, not one

StmtPrinter assumed that the first template arg was the pack and
attempted to iterate it.  However, the GNU extension (which is really
just N3599), has two template arguments.  In this case, the second
argument is the pack containing the string contents.

Handle this by desugaring the call to the explicit operator.

For example:
"qux" _zombocom will be shown as
operator "" _zombocom<char, 'q', 'u', 'x'>() in diagnostics and AST
dumps.

N.B.  It is actually impossible to render the arguments back to the
source form without storing more information in the AST.  For example,
we cannot tell if the user wrote u8"qux" or "qux".  We also lose
fidelity when it comes to non-char types for this exact reason (e.g. it
is hard to render a list of wchar_t back to something that can be
printed to the screen even if you don't have to consider surrogate
pairs).

This fixes PR23120.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234110 91177308-0d34-0410-b5e6-96231b3b80d8