Reworked the routine that qualifies the tool-chain for expected failures to handle sub-strings.
- For instance, allows 'gcc' to match x86-64-linux-gnu-gcc as required on some Debian builds.
- Also adds doc-strings and a more consistent naming convention for related helpers.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183415 91177308-0d34-0410-b5e6-96231b3b80d8