Introduce a fast path for the ASTReader's name lookup within a

Introduce a fast path for the ASTReader's name lookup within a
DeclContext. When the DeclContext is of a kind that can only be
defined once and never updated, we limit the search to the module file
that conatins the lookup table. Provides a 15% speedup in one
modules-heavy source file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173050 91177308-0d34-0410-b5e6-96231b3b80d8