[GlobalValue] Move HasLLVMReservedName into existing bitfield. NFC

Summary:
Follow-up to r290691, where I introduced HasLLVMReservedName.  rnk
pointed out that that patch added an extra word to GlobalValue on MSVC,
because it doesn't pack bitfields with different types.

This patch moves HasLLVMReservedName into the existing bitfield, where
we appear to have plenty of bits to spare.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28149

llvm-svn: 290696