[Bitcode] Add enums for call instruction markers and flags. NFC.

This commit adds enums in LLVMBitCodes.h to improve readability and
maintainability. This is a follow-up to r252368 which was discussed
here:

http://reviews.llvm.org/D12923


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252395 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed