Make Sema::LocallyScopedExternalDecls lazily deserialized. In theory,

Make Sema::LocallyScopedExternalDecls lazily deserialized. In theory,
we could turn this into an on-disk hash table so we don't load the
whole thing the first time we need it. However, it tends to be very,
very small (i.e., empty) for most precompiled headers, so it isn't all
that interesting.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136352 91177308-0d34-0410-b5e6-96231b3b80d8