Rollup merge of #57840 - tromey:fix-issue-57762, r=nikic
Rollup merge of #57840 - tromey:fix-issue-57762, r=nikic

Fix issue 57762

against 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 #57762

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust