ExpressionSourceCode.cpp is in source/Expressions - which should indicate that it is a generic part of the expression parser. In fact, almost everything it currently does is very Clang specific. This change factors out the clang specific bits into ClangExpressionSourceCode.cpp. It is an NFC change, but it will help to reduce conflicts in supporting other Expression Parsers (e.g. swift).
I am leaving ExpressionSourceCode.h in place even though at present it doesn't have much content. In the swift.org repository we have support for emitting debug information for compiled expressions - which requires saving out the expression text, and that is handled by this class. That's a sometimes handy feature. This change will make it easier to merge in that facility, though I'll do that as a separate commit.