IR: Rename LLVMContextImpl::MetadataStore to InstructionMetadata

Rename `MetadataStore` to the more explicit `InstructionMetadata`.  This
will make room for `FunctionMetadata` (start of PR23340).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235763 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed