[test][go]Use -stdlib=libc++ to build GO binding test if LLVM is built with libc++

[xingxue]

Summary

The recent change in go.test causes the GO binding test to fail if libc++ is used to build LLVM. When LLVM is built against libc++, LLVM libraries, including those used in GO binding test have dependencies on libc++ and C++ STL signatures in these libraries are corresponding to libc++ implementation. Therefore, -stdlib=libc++ is required on the C++ compiler command for building GO binding test that links with these LLVM libraries. Fixed by using --cxxflags as part of the CGO_CXXFLAGS and CGO_LDFLAGS configuration. --cxxflags has -srdlib=libc++ if libc++ is used to build LLVM.

[pcc]

This should be fixed by rG0414c5694073de26fd33a0276c47c6adea5284cf.

[xingxue]

Revision no longer needed now that the GO binding test code has been reverted. Thanks!

llvm/test/Bindings/Go/go.test

-; RUN: CGO_CPPFLAGS="`llvm-config --cppflags`" CGO_CXXFLAGS=-std=c++14 CGO_LDFLAGS="`llvm-config --ldflags --libs --system-libs all`" go test llvm
+; RUN: CGO_CPPFLAGS="`llvm-config --cppflags`" CGO_CXXFLAGS="`llvm-config --cxxflags`" CGO_LDFLAGS="`llvm-config --cxxflags --ldflags --libs --system-libs all`" go test llvm
 
 ; REQUIRES: shell
 ; UNSUPPORTED: asan, ubsan, msan