Create parameter for maximum number of template instantiations.

Currently, the maximum number is hard coded. This PR creates a command line parameter for it.
Requested by bearophile: http://forum.dlang.org/post/tsphogrgyjeqyeadqkds@forum.dlang.org

dmd2/mars.h

@@ -178,6 +178,7 @@ struct Param
     bool enforcePropertySyntax;
     bool addMain; // LDC_FIXME: Implement.
     bool allInst; // LDC_FIXME: Implement.
+    unsigned nestedTmpl; // maximum nested template instantiations
 #else
     bool pic;           // generate position-independent-code for shared libs
     bool cov;           // generate code coverage data

dmd2/template.c

@@ -5235,7 +5235,11 @@ void TemplateInstance::tryExpandMembers(Scope *sc2)
     static int nest;
     // extracted to a function to allow windows SEH to work without destructors in the same function
     //printf("%d\n", nest);
+#if IN_LLVM
+    if (++nest > global.params.nestedTmpl)
+#else
     if (++nest > 500)
+#endif
     {
         global.gag = 0;                 // ensure error message gets printed
         error("recursive expansion");
@@ -5705,7 +5709,11 @@ void TemplateInstance::semantic(Scope *sc, Expressions *fargs)
         while (ti && !ti->deferred && ti->tinst)
         {
             ti = ti->tinst;
+#if IN_LLVM
+            if (++nest > global.params.nestedTmpl)
+#else
             if (++nest > 500)
+#endif
             {
                 global.gag = 0;            // ensure error message gets printed
                 error("recursive expansion");
@@ -7530,7 +7538,11 @@ void TemplateMixin::semantic(Scope *sc)
 
     static int nest;
     //printf("%d\n", nest);
+#if IN_LLVM
+    if (++nest > global.params.nestedTmpl)
+#else
     if (++nest > 500)
+#endif
     {
         global.gag = 0;                 // ensure error message gets printed
         error("recursive expansion");

driver/cl_options.cpp

@@ -385,6 +385,12 @@ cl::opt<bool, true> allinst("allinst",
     cl::desc("generate code for all template instantiations"),
     cl::location(global.params.allInst));
 
+cl::opt<unsigned, true> nestedTemplateDepth("template-depth",
+    cl::desc("(experimental) set maximum number of nested template instantiations"),
+    cl::location(global.params.nestedTmpl),
+    cl::init(500));
+
+
 static cl::extrahelp footer("\n"
 "-d-debug can also be specified without options, in which case it enables all\n"
 "debug checks (i.e. (asserts, boundchecks, contracts and invariants) as well\n"

driver/cl_options.h

@@ -70,6 +70,8 @@ namespace opts {
     extern cl::opt<BoolOrDefaultAdapter, false, FlagParser> boundsChecks;
     extern bool nonSafeBoundsChecks;
 
+    extern cl::opt<unsigned, true> nestedTemplateDepth;
+
     // Arguments to -d-debug
     extern std::vector<std::string> debugArgs;
     // Arguments to -run