[PCH] After deserializing a DeclContext, if it has external lexical decls but not

[PCH] After deserializing a DeclContext, if it has external lexical decls but not
external visible decls, call DeclContext::setMustBuildLookupTable so that the
"lazy decls" bit of the LookupPtr is set.

Previously, in non-C++, if there were no new declarations causing the "lazy decls" bit
to be set, then DeclContext::lookups_begin() would fail to return the decls from the PCH.

Fixes rdar://12316296.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164351 91177308-0d34-0410-b5e6-96231b3b80d8