Add secondary libstdc++ 4.8 and 5.1 detection mechanisms

Summary:
The date-based approach to detecting unsupported versions of libstdc++
does not handle bug fix releases of older versions. As an example, the
`__GLIBCXX__` value associated with version 5.1, `20150422`, is less
than the values associated with versions 4.8.5 and 4.9.3.

This patch adds secondary checks based on certain properties in
sufficiently new versions of libstdc++.

Reviewers: jfb, tstellar, rnk, sfertile, nemanjai

Reviewed By: jfb

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58682

llvm-svn: 355638