[Clang] Enable IC/IF mode for __ibm128

As for 128-bit floating points on PowerPC, compiler should have three
machine modes:

- IFmode, always IBM extended double
- KFmode, always IEEE 754R 128-bit floating point
- TFmode, matches the semantics for long double

This commit adds support for IF mode with its complex variant, IC mode.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D109950