[Core] Remove use of ClangASTContext in DumpDataExtractor

Summary:
DumpDataExtractor uses ClangASTContext in order to get the proper llvm
fltSemantics for the type it needs so that it can dump floats in a more
precise way. However, there's no reason that this behavior needs to be
specific ClangASTContext. Instead, I think it makes sense to ask
TypeSystems for the float semantics for a type of a given size.

Differential Revision: https://reviews.llvm.org/D67239

llvm-svn: 371258