[clang] [Clang][AArch64] Remove unwarranted 'cannot be used in non-streaming mode' diagnostic. (PR #150592)

[Benjamin Maxwell via cfe-commits]

================
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sme -fsyntax-only -verify %s
+
+#include <arm_sme.h>
+
+void test_streaming(svint32_t *out, svint32_t *in) __arm_streaming {
+  *out = *in;
+}
+
+void test_non_streaming(svint32_t *out, svint32_t *in) {
+  *out = *in; // expected-error {{SVE vector type 'svint32_t' (aka '__SVInt32_t') cannot be used in a non-streaming function}} \
+                 expected-error {{SVE vector type 'svint32_t' (aka '__SVInt32_t') cannot be used in a non-streaming function}}
+}
+
+// This previously led to a diagnostic that '&a' could not be used in a non-streaming function,
+// even though all functions are streaming.
+void test_both_streaming(int32_t *out) __arm_streaming{
+  svint32_t a;
+  [&a, &out]() __arm_streaming {
+    a = svdup_s32(1);
+    svst1(svptrue_b32(), out, a);
+  }();
+}
+
+void test_lambda_streaming(int32_t *out) {
+  svint32_t a; // expected-error {{SVE vector type 'svint32_t' (aka '__SVInt32_t') cannot be used in a non-streaming function}}
+  [&a, &out]() __arm_streaming {
+    a = 1;
+    svst1(svptrue_b32(), out, a);
+  }();
+}
+
----------------
MacDue wrote:

For completeness, I think it would be good to test some non-streaming lambda cases:
```c++
// Error? Capture is by-reference and unused:
void test_lambda_non_streaming_capture() __arm_streaming {
  svint32_t a;
  [&a] {
    // Do nothing.
  }();
}


// Error: Non-streaming function attempts to dereference capture:  
void test_lambda_non_streaming_capture() __arm_streaming {              
  svint32_t a;                                                          
  [&a] {                                                                
    return a;                                                                 
  }();                                                                  
}                                

// Error? By reference capture, only records and uses the address of `a`:
void test_lambda_non_streaming_capture() __arm_streaming {
  svint32_t a;
  [&a] {
    return &a;
  }();
}                                       
  ``` 

https://github.com/llvm/llvm-project/pull/150592