Driver: implement --dyld-prefix option.

This option is used to select a dynamic loader prefix to be used
at runtime. Currently this is implemented for the Linux toolchain.

Differential Revision: http://llvm-reviews.chandlerc.com/D851

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182744 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index 1dbbc9a..5e8dde3 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -289,6 +289,8 @@
   }
   if (const Arg *A = Args->getLastArg(options::OPT__sysroot_EQ))
     SysRoot = A->getValue();
+  if (const Arg *A = Args->getLastArg(options::OPT__dyld_prefix_EQ))
+    DyldPrefix = A->getValue();
   if (Args->hasArg(options::OPT_nostdlib))
     UseStdLib = false;