Refactor the construction of nested-name-specifiers with

Refactor the construction of nested-name-specifiers with
source-location information into a NestedNameSpecifierLocBuilder
class, which lives within the AST library and centralize all knowledge
of the format of nested-name-specifier location information here.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126716 91177308-0d34-0410-b5e6-96231b3b80d8