Add a target triple to fix failures on MS build bots.

-ast-dump=json tries to mangle every declaration, and under the MS ABI
there are still a few things we can't mangle, which caused this test to
assert.

Author: zygoloid

clang/test/Coverage/ast-printing.cpp

@@ -4,7 +4,7 @@
 // RUN: diff %t.1.cpp %t.2.cpp
 // RUN: %clang_cc1 -std=c++20 -ast-dump %s
 // RUN: %clang_cc1 -std=c++20 -ast-dump-all %s
-// RUN: %clang_cc1 -std=c++20 -ast-dump=json %s
+// RUN: %clang_cc1 -std=c++20 -ast-dump=json -triple=x86_64-linux-gnu %s
 // RUN: %clang_cc1 -std=c++20 -fdump-record-layouts %s
 
 #include "cxx-language-features.inc"