[libcxx][AIX] Switch build compiler to clang

This patch switches the build compiler for AIX from ibm-clang to clang. ibm-clang++_r has `-pthread` by default, but clang for AIX doesn't, so `-pthread` had to be added to the test config. A bunch of tests now pass, so the `XFAIL` was removed. This patch also switch the build to use the visibility support available in clang-15 to control symbols exported by the shared library (AIX traditionally uses explicit export lists for this purpose).

Reviewed By: #libc, #libc_abi, daltenty, #libunwind, ldionne

Differential Revision: https://reviews.llvm.org/D127470

libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer.pass.cpp

@@ -6,8 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: LIBCXX-AIX-FIXME
-
 // <string>
 
 // basic_string<charT,traits,Allocator>& append(const charT* s); // constexpr since C++20