Partially revert r270816: build with -fvisibility=hidden.

This patch breaks the ABI on linux when libc++abi.a is statically linked into
libc++.so. Certain libc++ symbols get exported from libc++abi.a as hidden
and therefore they also get hidden in libc++.so.

The symbols is question are:

* _ZNKSt3__121__basic_string_commonILb1EE20__throw_out_of_rangeEv
* _ZNKSt3__121__basic_string_commonILb1EE20__throw_length_errorEv
* _ZNKSt3__120__vector_base_commonILb1EE20__throw_length_errorEv

I imagine we just need to fix the visibility for these symbols in the libc++
headers but I'm reverting the patch until it's sorted.

llvm-svn: 271500