[PowerPC] [Clang] Enable float128 feature on P9 by default

[qiucf]

Summary

As Power9 introduced hardware support for IEEE quad-precision FP type, the feature should be enabled by default on Power9 or newer targets.

And as I saw, GCC also enables the feature for all VSX-enabled targets (Power7 and Power8). If we want to align with GCC behaviors about FP128, we'd also do that after fixing f128 libcalls' calling convention issue.

[qiucf]

Ping..

[steven.zhang]

LGTM as it seems the missing part. Please also fix the error message if with Power8.

clang/lib/Basic/Targets/PPC.cpp

Features["vsx"] = llvm::StringSwitch<bool>(CPU)
                         .Case("pwr8", true)
                         .Case("pwr7", true)
                         .Default(false);
   Features["htm"] = llvm::StringSwitch<bool>(CPU)
                         .Case("ppc64le", true)
                         .Case("pwr9", true)
                         .Case("pwr8", true)
                         .Default(false);
+  Features["float128"] = llvm::StringSwitch<bool>(CPU)
+                        .Case("pwr9", true)
+                        .Default(false);
 
   Features["spe"] = llvm::StringSwitch<bool>(CPU)
                         .Case("8548", true)
                         .Case("e500", true)
                         .Default(false);
 
   // Power10 includes all the same features as Power9 plus any features specific
   // to the Power10 core.

clang/test/Driver/ppc-f128-support-check.c

 // RUN: not %clang -target powerpc64le-unknown-linux-gnu -fsyntax-only \
-// RUN:   -mcpu=pwr9 -mfloat128 %s 2>&1 | FileCheck %s --check-prefix=HASF128
+// RUN:   -mcpu=pwr9 %s 2>&1 | FileCheck %s --check-prefix=HASF128
 // RUN: not %clang -target powerpc64le-unknown-linux-gnu -fsyntax-only \
-// RUN:   -mcpu=power9 -mfloat128 %s 2>&1 | FileCheck %s --check-prefix=HASF128
+// RUN:   -mcpu=power9 %s 2>&1 | FileCheck %s --check-prefix=HASF128
 
 // RUN: not %clang -target powerpc64le-unknown-linux-gnu -fsyntax-only \
 // RUN:   -mcpu=pwr8 -mfloat128 %s 2>&1 | FileCheck %s --check-prefix=NOF128
 // RUN: not %clang -target powerpc64le-unknown-linux-gnu -fsyntax-only \
 // RUN:   -mcpu=pwr7 -mfloat128 %s 2>&1 | FileCheck %s --check-prefix=NOF128
 // RUN: not %clang -target powerpc64le-unknown-linux-gnu -fsyntax-only \
 // RUN:   -mfloat128 %s 2>&1 | FileCheck %s --check-prefix=NOF128
 
 #ifdef __FLOAT128__
 static_assert(false, "__float128 enabled");
 #endif
 
 // HASF128: __float128 enabled
 // HASF128-NOT: option '-mfloat128' cannot be specified with
 // NOF128: option '-mfloat128' cannot be specified with