[clang] Fix failing attribute test on Windows

The test added in r344146 was failing because the ABI on Windows is
different, and that test includes ABI-specific details. The test now
harcodes which ABI to use so we can rely on those details.

llvm-svn: 344159

clang/test/SemaCXX/attr-on-explicit-template-instantiation.cpp

-// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o - | FileCheck %s
 
 // PR39118
 // Make sure that attributes are properly applied to explicit template