[AST][NFC] Pack DependentScopeDeclRefExpr and CXXUnresolvedConstructExpr

Use the newly available space in the bit-fields of Stmt.
This saves 1 pointer per DependentScopeDeclRefExpr/CXXUnresolvedConstructExpr.

Additionally rename "TypeSourceInfo *Type;" to "TypeSourceInfo *TSI;"
as was done in D56022 (r350003) (but this is an internal detail anyway),
and clang-format both classes. NFC.

llvm-svn: 350525