[clang] Fix armv7-quick build by hardcoding -triple=x86_64 in OOM test.

[adamcz]

Summary

The test was added in 48bb5f4cbe8d5951c1153e469dc6713a122b7fa3 and it
creates a very large array, which is too large for ARM. Making the array
smaller is not a good option, since we would need a very low ulimit and
could then hit it for other reasons.

Should fix the https://lab.llvm.org/buildbot/#/builders/171/builds/5851
failure.

clang/test/SemaCXX/PR51712-large-array-constexpr-check-oom.cpp

 // REQUIRES: shell
 // UNSUPPORTED: win32
 // RUN: ulimit -v 1048576
-// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -triple=x86_64 %s
 // expected-no-diagnostics
 
 // This used to require too much memory and crash with OOM.
 struct {
   int a, b, c, d;
 } arr[1<<30];