[compiler-rt] Fix test failures when switching default C++ library

[Hahnfeld]

Summary

1. Add two explicit -stdlib=libstdc++ in conjunction with -static-libstdc++
2. Pass -nostdinc++ when adding include paths for libc++ built for tsan. This prevents clang finding the headers twice which would confuse #include_next

test/asan/TestCases/throw_invoke_test.cc

 // RUN: %clangxx_asan %s -o %t && %run %t
-// RUN: %clangxx_asan %s -o %t -static-libstdc++ && %run %t
+// RUN: %clangxx_asan %s -o %t -stdlib=libstdc++ -static-libstdc++ && %run %t
 
 // Clang doesn't support exceptions on Windows yet.
 // XFAIL: win32
 
 #include <stdio.h>
 static volatile int zero = 0;
 inline void pretend_to_do_something(void *x) {
   __asm__ __volatile__("" : : "r" (x) : "memory");

test/tsan/lit.cfg

 if config.has_libcxx and config.host_os != 'Darwin':
   # FIXME: Dehardcode this path somehow.
   libcxx_path = os.path.join(config.compiler_rt_obj_root, "lib",
                              "tsan", "libcxx_tsan_" + config.arch)
   libcxx_incdir = os.path.join(libcxx_path, "include", "c++", "v1")
   libcxx_libdir = os.path.join(libcxx_path, "lib")
   libcxx_so = os.path.join(libcxx_libdir, "libc++.so")
   clang_tsan_cxxflags += ["-std=c++11",
+                          "-nostdinc++",
                           "-I%s" % libcxx_incdir,
                           libcxx_so,
                           "-Wl,-rpath=%s" % libcxx_libdir]
 
 def build_invocation(compile_flags):
   return " " + " ".join([config.clang] + compile_flags) + " "
 
 config.substitutions.append( ("%clang_tsan ", build_invocation(clang_tsan_cflags)) )

test/tsan/static_init6.cc

-// RUN: %clangxx_tsan -static-libstdc++ -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -stdlib=libstdc++ -static-libstdc++ -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
 #include <pthread.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <sched.h>
 
 struct Cache {
   int x;
   explicit Cache(int x)