Fix another case where libstdc++ is being inappropriately requested (see

Fix another case where libstdc++ is being inappropriately requested (see
r328775) for all platforms.

Given that this is the second occurance of this problem it seemed worth
fixing this problem in a more generic way. r328775 has been reverted and
now a substitution `%linux_static_libstdcplusplus` has been provided.
This substitution expands to Clang driver arguments to use a static
libstdc++ on Linux and on all other platforms it expands to nothing.

The `asan/TestCases/throw_invoke_test.cc` and
`test/tsan/static_init6.cc` test cases now use this substitution.

rdar://problem/39948818

Differential Revision: https://reviews.llvm.org/D46401

llvm-svn: 332254