Fix issue 57762

Issue 57762 points out a compiler crash when the compiler was built
using a stock LLVM 7.  LLVM 7 was released without a necessary fix for
a bug in the DWARF discriminant code.

This patch changes rustc to use the fallback mode on (non-Rust) LLVM 7.

Closes rust-lang#57762

src/librustc_codegen_llvm/debuginfo/metadata.rs

@@ -1164,7 +1164,11 @@ fn use_enum_fallback(cx: &CodegenCx) -> bool {
     // On MSVC we have to use the fallback mode, because LLVM doesn't
     // lower variant parts to PDB.
     return cx.sess().target.target.options.is_like_msvc
-        || llvm_util::get_major_version() < 7;
+        || llvm_util::get_major_version() < 7
+        // LLVM version 7 did not release with an important bug fix;
+        // but the required patch is in the equivalent Rust LLVM.
+        // See https://github.com/rust-lang/rust/issues/57762.
+        || (llvm_util::get_major_version() == 7 && unsafe { !llvm::LLVMRustIsRustLLVM() });
 }
 
 // Describes the members of an enum value: An enum is described as a union of