ToolChains: Minor touchup to use correct type, avoid truncation.

Truncation happens regularly when find_first_not_of returns npos,
strings long enough to trigger bug here are implausible.

No functionality change intended (ignoring absurd string lengths).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172127 91177308-0d34-0410-b5e6-96231b3b80d8