Pass an ArgList to every toolchain constructor. Remove the useIntegratedAs
argument.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177301 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused argument.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177303 91177308-0d34-0410-b5e6-96231b3b80d8
Use early returns when checking if we already constructed a tool and when
delegating to Generic_GCC::SelectTool (it already updates the tool map).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177305 91177308-0d34-0410-b5e6-96231b3b80d8