[clang] Satisfy clang v12

Older versions of clang (for example, v12) throw an error when compiling
CStringChecker.cpp that the initializers for `SourceArgExpr`,
`DestinationArgExpr`, and `SizeArgExpr` are missing braces around
initialization of subobject.  Newer clang versions don't throw this
error.  This patch adds the initialization braces to satisfy clang.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D154871