[clang] Fix -fclang-abi-compat for clang 20 (llvm#144109)

The value was known already, but it was parsed as latest which is
incorrect because we are already doing clang 21.

Author: Fznamznon

clang/lib/Frontend/CompilerInvocation.cpp

@@ -4475,6 +4475,8 @@ bool CompilerInvocation::ParseLangArgs(LangOptions &Opts, ArgList &Args,
         Opts.setClangABICompat(LangOptions::ClangABI::Ver18);
       else if (Major <= 19)
         Opts.setClangABICompat(LangOptions::ClangABI::Ver19);
+      else if (Major <= 20)
+        Opts.setClangABICompat(LangOptions::ClangABI::Ver20);
     } else if (Ver != "latest") {
       Diags.Report(diag::err_drv_invalid_value)
           << A->getAsString(Args) << A->getValue();

clang/test/CodeGen/X86/avx-cxx-record.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 %s -triple x86_64-unknown-linux-gnu -emit-llvm -O2 -target-cpu x86-64-v3 -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple x86_64-unknown-linux-gnu -emit-llvm -O2 -target-cpu x86-64-v3 -fclang-abi-compat=20 -o - | FileCheck --check-prefix CLANG-20 %s
 
 using UInt64x2 = unsigned long long __attribute__((__vector_size__(16), may_alias));
 
@@ -11,6 +12,7 @@ struct XMM2 : XMM1<0>, XMM1<1> {
 };
 
 // CHECK: define{{.*}} @_Z3foov({{.*}} [[ARG:%.*]]){{.*}}
+// CLANG-20: define{{.*}} <4 x double> @_Z3foov()
 // CHECK: entry:
 // CHECK-NEXT: store {{.*}}, ptr [[ARG]]{{.*}}
 // CHECK-NEXT: [[TMP1:%.*]] = getelementptr {{.*}}, ptr [[ARG]]{{.*}}