Support unwinding from inline assembly

I've taken the following steps to add unwinding support from inline assembly:

1) Add a new `unwind` "attribute" (like `sideeffect`) to the asm syntax:

```
invoke void asm sideeffect unwind "call thrower", "~{dirflag},~{fpsr},~{flags}"()
    to label %exit unwind label %uexit
```

2.) Add Bitcode writing/reading support + LLVM-IR parsing.

3.) Emit EHLabels around inline assembly lowering (SelectionDAGBuilder + GlobalISel) when `InlineAsm::canThrow` is enabled.

4.) Tweak InstCombineCalls/InlineFunction pass to not mark inline assembly "calls" as nounwind.

5.) Add clang support by introducing a new clobber: "unwind", which lower to the `canThrow` being enabled.

6.) Don't allow unwinding callbr.

Reviewed By: Amanieu

Differential Revision: https://reviews.llvm.org/D95745

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -2429,9 +2429,9 @@ void ModuleBitcodeWriter::writeConstants(unsigned FirstVal, unsigned LastVal,
     }
 
     if (const InlineAsm *IA = dyn_cast<InlineAsm>(V)) {
-      Record.push_back(unsigned(IA->hasSideEffects()) |
-                       unsigned(IA->isAlignStack()) << 1 |
-                       unsigned(IA->getDialect()&1) << 2);
+      Record.push_back(
+          unsigned(IA->hasSideEffects()) | unsigned(IA->isAlignStack()) << 1 |
+          unsigned(IA->getDialect() & 1) << 2 | unsigned(IA->canThrow()) << 3);
 
       // Add the asm string.
       const std::string &AsmStr = IA->getAsmString();