[RISCV] Disallow target attribute use in multiversioning (#85899)

For RISC-V target only `target_clones` and `target_version` can enable
function multiversion(FMV).

This patch make target attribute trigger redefinition instead of emit
FMV.

Here is spec
https://github.com/riscv-non-isa/riscv-c-api-doc/blob/master/riscv-c-api.md#__attribute__targetattr-string

clang/lib/Sema/SemaDecl.cpp

@@ -11911,8 +11911,14 @@ static bool CheckMultiVersionFunction(Sema &S, FunctionDecl *NewFD,
     return false;
   }
 
+  const llvm::Triple &T = S.getASTContext().getTargetInfo().getTriple();
+
   // Target attribute on AArch64 is not used for multiversioning
-  if (NewTA && S.getASTContext().getTargetInfo().getTriple().isAArch64())
+  if (NewTA && T.isAArch64())
+    return false;
+
+  // Target attribute on RISCV is not used for multiversioning
+  if (NewTA && T.isRISCV())
     return false;
 
   if (!OldDecl || !OldDecl->getAsFunction() ||

clang/test/Sema/attr-target-riscv.c

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -triple riscv64-linux-gnu -target-feature +i -fsyntax-only -verify -std=c2x %s
+
+//expected-note@+1 {{previous definition is here}}
+int __attribute__((target("arch=rv64g"))) foo(void) { return 0; }
+//expected-error@+1 {{redefinition of 'foo'}}
+int __attribute__((target("arch=rv64gc"))) foo(void) { return 0; }