Move StringLiteral to allocate its internal string data using the allocator in

Move StringLiteral to allocate its internal string data using the allocator in
ASTContext. This required changing all clients to pass in the ASTContext& to the
constructor of StringLiteral. I also changed all allocations of StringLiteral to
use new(ASTContext&).

Along the way, I updated a bunch of new()'s in StmtSerialization.cpp to use the
allocator from ASTContext& (not complete).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63958 91177308-0d34-0410-b5e6-96231b3b80d8