Support build and target configurations

These changes add support for build and target configurations in the compiler.
Build and target configurations, combined with the use of #if/#else/#endif allow
for conditional compilation within declaration and statement contexts.

Build configurations can be passed into the compiler via the new '-D' flag, or
set within the LangOptions class. Target configurations are implicit, and
currently only "os" and "arch" are supported.

Swift SVN r14305

lib/Frontend/CompilerInvocation.cpp

@@ -512,6 +512,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
     Opts.DebugConstraintSolverAttempt = attempt;
   }
+  
+  for (const Arg *A : make_range(Args.filtered_begin(OPT_D),
+                                 Args.filtered_end())) {
+    Opts.BuildConfigOptions[A->getValue()] = A->getValue();
+  }
 
   return false;
 }