AST: defer to TypeLoc::copy in TypeLoc::initializeFullCopy

AST: defer to TypeLoc::copy in TypeLoc::initializeFullCopy

If we're initializing a TypeLoc from one that's been allocated with
different alignment, memcpy will get the padding wrong. The `copy`
method already checks and handles this case, so we should just defer
to it.

This also drops the `const` off of the `initializeFullCopy`
declarations, since it isn't even remotely true (and the compiler
notices when we try to call copy() instead of tricking it with
memcpy).

Fixes llvm.org/pr23516.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254935 91177308-0d34-0410-b5e6-96231b3b80d8