Always use the ThinLTO pipeline for pre-link optimizations

When using cargo this was already effectively done for all dependencies
as cargo passes -Clinker-plugin-lto without -Clto=fat/thin.
-Clinker-plugin-lto assumes that ThinLTO will be used. The ThinLTO
pre-link pipeline is faster than the fat LTO one. And according to the
benchmarks in [1] there is barely any runtime performance difference
between executables that used fat LTO with the fat vs ThinLTO pre-link
pipeline.

[1]: https://discourse.llvm.org/t/rfc-a-unified-lto-bitcode-frontend/61774

Author: bjorn3

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -840,11 +840,8 @@ extern "C" LLVMRustResult LLVMRustOptimize(
         }
         break;
       case LLVMRustOptStage::PreLinkThinLTO:
-        MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
-        NeedThinLTOBufferPasses = false;
-        break;
       case LLVMRustOptStage::PreLinkFatLTO:
-        MPM = PB.buildLTOPreLinkDefaultPipeline(OptLevel);
+        MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
         NeedThinLTOBufferPasses = false;
         break;
       case LLVMRustOptStage::ThinLTO: