[modules] Further simplification and speedup of redeclaration chain loading.

[modules] Further simplification and speedup of redeclaration chain loading.

Instead of eagerly deserializing a list of DeclIDs when we load a module file
and doing a binary search to find the redeclarations of a decl, store a list of
redeclarations of each chain before the first declaration and load it directly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245789 91177308-0d34-0410-b5e6-96231b3b80d8