Fix complex types declared using mode TC

This patch reverts incorrect IR introduced in commit d11ec6f6
[Clang] Enable IC/IF mode for __ibm128, for complex types declared
using __attribute__((mode(TC))). TC corresponds to an unspecified
128-bit format, which on some targets is a double-double format
(like __ibm128_t) and on others is float128_t. The bug in d11ec6f6
is that long double is only safe to use when it's known to be one of
these formats.

Differential Revision: https://reviews.llvm.org/D112975