[yaml2obj] - Make interface of `NameToIdxMap` class be human friendly and fix users.
[yaml2obj] - Make interface of `NameToIdxMap` class be human friendly and fix users.

This patch inverses the values returned by `addName` and
`lookup` methods of the class mentioned so that they
now return true on success and false on failure.
Also, it does minor code cleanup.

Differential revision: https://reviews.llvm.org/D61190

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@359818 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm