[analyzer] Turn suppress-c++-stdlib on by default

[analyzer] Turn suppress-c++-stdlib on by default

We have several reports of false positives coming from libc++. For example,
there are reports of false positives in std::regex, std::wcout, and also
a bunch of issues are reported in https://reviews.llvm.org/D30593. In many
cases, the analyzer trips over the complex libc++ code invariants. Let's turn
off the reports coming from these headers until we can re-evalate the support.

We can turn this back on once we individually suppress all known false
positives and perform deeper evaluation on large codebases that use libc++.
We'd also need to commit to doing these evaluations regularly as libc++
headers change.

Differential Revision: https://reviews.llvm.org/D30798

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297429 91177308-0d34-0410-b5e6-96231b3b80d8