Avoid dubious IdentifierInfo::getNameStart() uses

These cases in particular were incurring an extra strlen() when we already knew
the length. They appear to be leftovers from when the interfaces worked with C
strings that have continued to compile due to the implicit StringRef ctor.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210403 91177308-0d34-0410-b5e6-96231b3b80d8