Revert "Set opt-level to 3"

This reverts commit aad9840ad4.

Level 3 (possibly indirectly, the underlying bug might be in XCode’s linker)
causes unit tests to sefault when compiled with some versions of XCode:
https://github.com/rust-lang/rust/issues/50867

It also appears to cause some segfaults on Windows:
https://github.com/rust-lang/rust/pull/50329#issuecomment-386853473

… and regressions in some rustc performance benchmarks:
https://github.com/rust-lang/rust/pull/50329#issuecomment-388084894

src/Cargo.toml

@@ -40,6 +40,14 @@ members = [
   "tools/rls/test_data/workspace_symbol",
 ]
 
+# Curiously, compiletest will segfault if compiled with opt-level=3 on 64-bit
+# MSVC when running the compile-fail test suite when a should-fail test panics.
+# But hey if this is removed and it gets past the bots, sounds good to me.
+[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]