[Coverage] Get rid of an input/output parameter (NFC)
[Coverage] Get rid of an input/output parameter (NFC)

readFunctionRecords is used to iterate through the entries of the
coverage mapping section. Instead of expecting the function to update
the iterator through a `const char *&` parameter, just return the
updated iterator.

This will help us experiment with zlib-compressing coverage mapping
data.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@273052 91177308-0d34-0410-b5e6-96231b3b80d8

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