Support C++ friend declarations for PCH.

Support C++ friend declarations for PCH.

This commit 'introduces' a slightly different way to restore the state of the AST object.
It makes PCHDeclReader/PCHDeclWriter friends and gives them access to the private members of the object.
The rationale is to avoid using/modifying the AST interfaces for PCH read/write so that to:

-Avoid complications with objects that have side-effects during creation or when using some setters.
-Not 'pollute' the AST interface with methods only used by the PCH reader/writer
-Allow AST objects to be read-only.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107219 91177308-0d34-0410-b5e6-96231b3b80d8