Fix complex types declared using mode TC

This patch reverts incorrect IR introduced in commit d11ec6f
[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 d11ec6f
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

Author: elizabethandrews

clang/lib/Basic/TargetInfo.cpp

@@ -300,8 +300,11 @@ FloatModeKind TargetInfo::getRealTypeByWidth(unsigned BitWidth,
     if (ExplicitType == FloatModeKind::Ibm128)
       return hasIbm128Type() ? FloatModeKind::Ibm128
                              : FloatModeKind::NoFloat;
-    if (ExplicitType == FloatModeKind::LongDouble)
-      return ExplicitType;
+    if (&getLongDoubleFormat() == &llvm::APFloat::PPCDoubleDouble() ||
+        &getLongDoubleFormat() == &llvm::APFloat::IEEEquad())
+      return FloatModeKind::LongDouble;
+    if (hasFloat128Type())
+      return FloatModeKind::Float128;
     break;
   }
 

clang/test/CodeGenCXX/complex128.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -emit-llvm -triple x86_64-unknown-linux-gnu %s -o - | FileCheck %s
+
+// Define __complex128 type corresponding to __float128 (as in GCC headers).
+typedef _Complex float __attribute__((mode(TC))) __complex128;
+
+void check() {
+  // CHECK: alloca { fp128, fp128 }
+  __complex128 tmp;
+}