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).

llvm-svn: 63958