Auto merge of rust-lang#81451 - nikic:llvm-12, r=nagisa

Upgrade to LLVM 12

This implements the necessary adjustments to make rustc work with LLVM 12. I didn't encounter any major issues so far.

r? `@cuviper`

Author: bors

profiler_builtins/build.rs

@@ -26,6 +26,7 @@ fn main() {
         "InstrProfilingPlatformWindows.c",
         "InstrProfilingUtil.c",
         "InstrProfilingValue.c",
+        "InstrProfilingVersionVar.c",
         "InstrProfilingWriter.c",
         // This file was renamed in LLVM 10.
         "InstrProfilingRuntime.cc",