[opt] Change the parameter of OptTable::PrintHelp from Name to Usage and don't...

[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