[BOLT][NFC] Fix undefined behavior in encodeAnnotationImm

Fix UBSan-reported issue in MCPlusBuilder::encodeAnnotationImm (left shift of a
negative value).

Test Plan:
```
ninja check-bolt
...
PASS: BOLT-Unit :: Core/./CoreTests/AArch64/MCPlusBuilderTester.Annotation/0 (1 of 140)
PASS: BOLT-Unit :: Core/./CoreTests/X86/MCPlusBuilderTester.Annotation/0 (131 of 134)
```

Reviewed By: maksfb, yota9

Differential Revision: https://reviews.llvm.org/D120260