Simplify memory management in NestedNameSpecifierLocBuilder.

With this change the memory of buffer in NestedNameSpecifierLocBuilder
is allocated in one place. It also prevents from allocation of tiny blocks.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213178 91177308-0d34-0410-b5e6-96231b3b80d8