Document option '-rtlib' in clang's man page and help info

This patch adds an entry for "-rtlib" in the output of `man clang` and `clang -help`.

Patch by Lei Zhang!

Differential Revision: https://reviews.llvm.org/D24069

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281440 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/clang.rst

@@ -105,7 +105,12 @@ Language Selection and Mode Options
 .. option:: -stdlib=<library>
 
  Specify the C++ standard library to use; supported options are libstdc++ and
- libc++.
+ libc++. If not specified, platform default will be used.
+
+.. option:: -rtlib=<library>
+
+ Specify the compiler runtime library to use; supported options are libgcc and
+ compiler-rt. If not specified, platform default will be used.
 
 .. option:: -ansi
 

include/clang/Driver/Options.td

@@ -1863,7 +1863,8 @@ def resource_dir : Separate<["-"], "resource-dir">,
 def resource_dir_EQ : Joined<["-"], "resource-dir=">, Flags<[DriverOption]>,
   Alias<resource_dir>;
 def rpath : Separate<["-"], "rpath">, Flags<[LinkerInput]>;
-def rtlib_EQ : Joined<["-", "--"], "rtlib=">;
+def rtlib_EQ : Joined<["-", "--"], "rtlib=">,
+  HelpText<"Compiler runtime library to use">;
 def r : Flag<["-"], "r">, Flags<[LinkerInput,NoArgumentUnused]>;
 def save_temps_EQ : Joined<["-", "--"], "save-temps=">, Flags<[DriverOption]>,
   HelpText<"Save intermediate compilation results.">;