Fix macos target assumption in test

Clang test Driver/macos-apple-silicon-slice-link-libs-darwin-only.cpp
assumes the target is darwin when the host is darwin which is not
necessarily the case, causing the test to fail when it is not. This
commit adds a -triple argument to the clang invocation to ensure the
target is darwin.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D94396

clang/test/Driver/macos-apple-silicon-slice-link-libs-darwin-only.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang -### -arch arm64 -mmacosx-version-min=10.7 %s 2>&1 | FileCheck -check-prefix=ARM64-10_7 %s
-// RUN: %clang -### -arch x86_64 -mmacosx-version-min=10.7 %s 2>&1 | FileCheck -check-prefix=x86_64-10_7 %s
+// RUN: %clang -### -target arm64-apple-darwin -arch arm64 -mmacosx-version-min=10.7 %s 2>&1 | FileCheck -check-prefix=ARM64-10_7 %s
+// RUN: %clang -### -target x86_64-apple-darwin10 -arch x86_64 -mmacosx-version-min=10.7 %s 2>&1 | FileCheck -check-prefix=x86_64-10_7 %s
 // REQUIRES: system-darwin
 
 // ARM64-10_7-NOT: -lcrt1.10.6.o