Implement intrinsic mangling for literal struct types.

Fixes PR 31921

Summary:
Predicateinfo requires an ugly workaround to try to avoid literal
struct types due to the intrinsic mangling not being implemented.
This workaround actually does not work in all cases (you can hit the
assert by bootstrapping with -print-predicateinfo), and can't be made
to work without DFS'ing the type (IE copying getMangledStr and using a
version that detects if it would crash).

Rather than do that, i just implemented the mangling.  It seems
simple, since they are unified structurally.

Looking at the overloaded-mangling testcase we have, it actually turns
out the gc intrinsics will *also* crash if you try to use a literal
struct.  Thus, the testcase added fails before this patch, and works
after, without needing to resort to predicateinfo.

Reviewers: chandlerc, davide

Subscribers: llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D29925

llvm-svn: 295253