[Clang] Fix failing CI with different test case attribute & host trip…

…le (#76863)

As in title,
a8f4397
broke CI due to the calling convention not available on certain targets.
This patch uses a simpler calling convention and enables the test only
when the attribute exists. It's verified that this test crashes the
compiler before a8f4397 so it's the
same effect as the previous test. Disabling the test on platforms that
don't have the calling convention is fine because it's guarding against
a frontend bug.

clang/test/SemaCXX/template-instantiation.cpp

@@ -1,11 +1,11 @@
-// RUN: %clang_cc1 -verify -fsyntax-only -Wno-ignored-attributes %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -verify -fsyntax-only %s
 // expected-no-diagnostics
 
 namespace GH76521 {
 
 template <typename T>
 void foo() {
-  auto l = []() __attribute__((pcs("aapcs-vfp"))) {};
+  auto l = []() __attribute__((preserve_most)) {};
 }
 
 void bar() {