[CommandLine] Error message for incorrect PositionalEatArgs usage

Summary:
bugpoint has several options specified as `PositionalEatArgs` to pass
options through to the underlying tool, e.g. `-tool-args`. The `-help`
message suggests the usage is: `-tool-args=<string>`. However, this is
misleading, because that's not how these arguments work. Rather than taking
a value, the option consumes all positional arguments until the next
recognized option (or all arguments if `--` is specified at some point).
To make this slightly clearer, instead print the help as:
```
  -tool-args <string>...                            - <tool arguments>...
```
Additionally, add an error if the user attempts to use a `PositionalEatArgs`
argument with a value, instead of silently ignoring it. Example:
```
./bin/bugpoint -tool-args=-mpcu=skylake-avx512
bugpoint: for the -tool-args option: This argument does not take a value.
    Instead, it consumes any positional arguments until the next recognized option.
```

Reviewed By: aprantl
Differential Revision: https://reviews.llvm.org/D46787

llvm-svn: 332311

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/include/llvm/Support/CommandLine.h b/llvm/include/llvm/Support/CommandLine.h
index ffd1d30ca3a..799b41fbf8b 100644
--- a/llvm/include/llvm/Support/CommandLine.h
+++ b/llvm/include/llvm/Support/CommandLine.h
@@ -30,6 +30,7 @@
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ManagedStatic.h"
+#include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <climits>
 #include <cstddef>
@@ -362,7 +363,10 @@ public:
                              bool MultiArg = false);
 
   // Prints option name followed by message.  Always returns true.
-  bool error(const Twine &Message, StringRef ArgName = StringRef());
+  bool error(const Twine &Message, StringRef ArgName = StringRef(), raw_ostream &Errs = llvm::errs());
+  bool error(const Twine &Message, raw_ostream &Errs) {
+    return error(Message, StringRef(), Errs);
+  }
 
   inline int getNumOccurrences() const { return NumOccurrences; }
   inline void reset() { NumOccurrences = 0; }