Revert "Auto merge of #52212 - kennytm:o3-again, r=alexcrichton"

This reverts commit 1731f0af22, reversing
changes made to 0a8275f8b6.

src/Cargo.toml

@@ -40,6 +40,13 @@ members = [
   "tools/rls/test_data/workspace_symbol",
 ]
 
+# Curiously, LLVM 7.0 will segfault if compiled with opt-level=3
+# See issue https://github.com/rust-lang/rust/issues/52378
+[profile.release]
+opt-level = 2
+[profile.bench]
+opt-level = 2
+
 # These options are controlled from our rustc wrapper script, so turn them off
 # here and have them controlled elsewhere.
 [profile.dev]