[Driver] Simplify ToolChain::GetCXXStdlibType (NFC)

I made the wrong assumption that execution would continue after an error Diag
which led to unneeded complex code.
This patch aligns with the better implementation of ToolChain::GetRuntimeLibType.

Differential Revision: https://reviews.llvm.org/D25669

llvm-svn: 289422