Fix ambiguous resolution of clang::ArrayType/llvm::ArrayType in ClangAstContext

Both llvm and clang have an ArrayType class, which can cause resolution to fail when llvm headers that are implicitly included name this type.
source/Symbol/ClangASTContext.cpp has 'using namespace llvm;' and 'using namespace clang;' 

Author: Luke Drummond <luke.drummond@codeplay.com>
Differential Revision: http://reviews.llvm.org/D16155

llvm-svn: 257759