[BOLT][NFC] Fix MCPlusBuilder::getAliases caching behavior

Caching behavior of `getAliases` causes a failure in unit tests where two
MCPlusBuilder objects are created corresponding to AArch64 and X86:
the alias cache is created for AArch64 but then used for X86.

https://lab.llvm.org/staging/#/builders/211/builds/126

The issue only affects unit tests as we only construct one MCPlusBuilder
for ELF binary.

Resolve the issue by moving alias bitvectors to MCPlusBuilder object.

Reviewed By: yota9

Differential Revision: https://reviews.llvm.org/D124942