[opt] Change the parameter of OptTable::PrintHelp from Name to Usage and don't append "[options] <inputs>"

Summary:
Before, "[options] <inputs>" is unconditionally appended to the `Name` parameter. It is more flexible to change its semantic to `Usage` and let user customize the usage line.

% llvm-objcopy
...
USAGE: llvm-objcopy <input> [ <output> ] [options] <inputs>

With this patch:

% llvm-objcopy
...
USAGE: llvm-objcopy input [output]

Reviewers: rupprecht, alexshap, jhenderson

Reviewed By: rupprecht

Subscribers: jakehehrlich, mehdi_amini, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D51009

llvm-svn: 344097
diff --git a/llvm/unittests/Option/OptionParsingTest.cpp b/llvm/unittests/Option/OptionParsingTest.cpp
index eef21ab..230049c 100644
--- a/llvm/unittests/Option/OptionParsingTest.cpp
+++ b/llvm/unittests/Option/OptionParsingTest.cpp
@@ -97,6 +97,10 @@
   T.PrintHelp(RSO, "test", "title!");
   EXPECT_NE(std::string::npos, Help.find("-A"));
 
+  // Check usage line.
+  T.PrintHelp(RSO, "name [options] file...", "title!");
+  EXPECT_NE(std::string::npos, Help.find("USAGE: name [options] file...\n"));
+
   // Test aliases.
   auto Cs = AL.filtered(OPT_C);
   ASSERT_NE(Cs.begin(), Cs.end());