Use LLVM-style command line (instead of DMD-style)

Note: For a backward compatible interface, use the new bin/ldmd script. It
      supports all old options while passing on anything it doesn't recognize.

Some changes caused by this:
* -debug and -version are now -d-debug and -d-version due to a conflict with
  standard LLVM options.
* All "flag" options now allow an optional =true/=1/=false/=0 suffix.
* Some "hidden debug switches" starting with "--" were renamed because LLVM
  doesn't care about the number of dashes, so they were conflicting with other
  options (such as -c).
  The new versions start with "-hidden-debug-" instead of "--"
* --help works, but has a non-zero exit code. This breaks some Tango scripts
  which use it to test for compiler existence. See tango.patch.

Some changes not (directly) caused by this;
* (-enable/-disable)-FOO options are now available for pre- and postconditions.
* -march is used instead of -m (like other LLVM programs), but -m is an alias
  for it.
* -defaultlib, -debuglib, -d-debug and -d-version allow comma-separated values.
  The effect should be identical to specifying the same option multiple times.
  I decided against allowing these for some other options because paths might
  contain commas on some systems.
* -fPIC is removed in favor of the standard LLVM option -relocation-model=pic

Bug:
* If -run is specified as the last argument in DFLAGS, no error is generated.
  (Not very serious IMHO)

gen/logger.cpp

@@ -8,6 +8,7 @@
 
 #include "mars.h"
 
+#include "llvm/Support/CommandLine.h"
 #include "gen/logger.h"
 
 namespace Logger
@@ -15,7 +16,10 @@ namespace Logger
     static std::string indent_str;
     static std::ofstream null_out("/dev/null");
 
-    static bool _enabled = false;
+    llvm::cl::opt<bool> _enabled("vv",
+        llvm::cl::desc("Very verbose"),
+        llvm::cl::ZeroOrMore);
+
     void indent()
     {
         if (_enabled) {