LDC: Apply dflags affecting symbol visibility to all deps

Example scenarios on Windows:

* If an .exe is linked against druntime/Phobos DLLs (e.g., to enable
  loading other D DLLs, all sharing central druntime/Phobos), all
  static-lib dub dependencies need to be compiled with
  `-dllimport=defaultLibsOnly` too [the default with
  `-link-defaultlib-shared`].
* To make large DLLs with few selective `export`ed symbols work after #2412,
  the implicit flags need to be overridden - for the DLL itself and
  all its static-lib dependencies. This can currently only be
  accomplished by setting the DFLAGS environment variable. This PR
  enables overriding these `dflags` either in the DLL root project/
  config directly, or in some dependency shared by multiple DLLs/
  executables.
  E.g., at Symmetry, we have an .exe with lots of plugin DLLs. All
  plugins and the .exe have a shared dependency, so adding
  `dflags "-fvisibility=hidden" "-dllimport=defaultLibsOnly" platform="windows-ldc"`
  once in the dub.sdl of that shared dependency suffices to compile
  *everything* with those required flags.

source/dub/generators/generator.d

@@ -459,6 +459,39 @@ class ProjectGenerator
 		configureDependents(*roottarget, targets);
 		visited.clear();
 
+		// LDC: need to pass down dflags affecting symbol visibility, especially on Windows
+		if (genSettings.platform.compiler == "ldc")
+		{
+			const isWindows = genSettings.platform.isWindows();
+			bool passDownDFlag(string flag)
+			{
+				if (flag.startsWith("--"))
+					flag = flag[1 .. $];
+				return flag.startsWith("-fvisibility=") || (isWindows &&
+					(flag.startsWith("-link-defaultlib-shared") ||
+					 flag.startsWith("-dllimport=")));
+			}
+
+			// all dflags from dependencies have already been added to the root project
+			auto rootDFlagsToPassDown = roottarget.buildSettings.dflags.filter!passDownDFlag.array;
+
+			if (rootDFlagsToPassDown.length)
+			{
+				foreach (name, ref ti; targets)
+				{
+					if (&ti != roottarget)
+					{
+						import std.range : chain;
+						ti.buildSettings.dflags = ti.buildSettings.dflags
+							// remove all existing visibility flags first to reduce duplicates
+							.filter!(f => !passDownDFlag(f))
+							.chain(rootDFlagsToPassDown)
+							.array;
+					}
+				}
+			}
+		}
+
 		// 4. As an extension to configureDependents we need to copy any injectSourceFiles
 		// in our dependencies (ignoring targetType)
 		void configureDependentsFinalImages(ref TargetInfo ti, TargetInfo[string] targets, ref TargetInfo finalBinaryTarget, size_t level = 0)