Accommodate empty string for build type. This was previously causing an error when generating an Xcode project using the CMake files (thanks to Doug Gregor for identifying the issue).
Accommodate empty string for build type. This was previously causing an error
when generating an Xcode project using the CMake files (thanks to Doug Gregor
for identifying the issue).


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@68618 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,chubbymaggie/asap,chubbymaggie/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,dslab-epfl/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,chubbymaggie/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap,dslab-epfl/asap,dslab-epfl/asap,dslab-epfl/asap,apple/swift-llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,chubbymaggie/asap,chubbymaggie/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm