Implement more efficient Decl <-> DeclContext conversions.

Implement more efficient Decl <-> DeclContext conversions.
When the static type on the Decl side is a subclass of DeclContext the compiler will use a "inlinable" static_cast, instead of always using an out-of-line function call.

Note, though, that the isa<> check still uses an out-of-line function call.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57415 91177308-0d34-0410-b5e6-96231b3b80d8