[clang] [Clang] disallow selectany on non-global-variable declarations (PR #189641)

[Aaron Ballman via cfe-commits]

================
@@ -53,3 +53,14 @@ extern const SomeStruct some_struct;
 
 // Without selectany, this should stay an error.
 const SomeStruct some_struct2; // expected-error {{default initialization of an object of const type 'const SomeStruct' without a user-provided default constructor}}
+
+struct __declspec(selectany) S1 {}; // expected-error {{'selectany' attribute only applies to global variables}}
+__declspec(selectany) struct S1 s1;
+
+void t() {
+  __declspec(selectany) int x; // expected-error {{'selectany' attribute only applies to global variables}}
+  __declspec(selectany) extern int y;
----------------
AaronBallman wrote:

Please add:
```
__declspec(selectany) static int z;
```
as I think using `GlobalVar` will cause this to succeed but we want it diagnosed.

https://github.com/llvm/llvm-project/pull/189641