[bash-autocompletion] Fix bug when a flag ends with '='

There was a bug that when a flag ends with '=' and no value was suggested,
clang autocompletes the flag itself.
For example, in bash, it looked like this:
```
$ clang -fmodule-file=[tab]
-> $clang -fmodule-file=-fmodule-file
```
This is not what we expect. We expect a file autocompletion when no value
was found. With this patch, pressing tab suggests files in the current
directory.

Reviewers: teemperor, ruiu

Subscribers: cfe-commits
llvm-svn: 345121

clang/lib/Driver/Driver.cpp

@@ -1534,7 +1534,9 @@ void Driver::HandleAutocompletions(StringRef PassedFlags) const {
   if (SuggestedCompletions.empty())
     SuggestedCompletions = Opts->suggestValueCompletions(Cur, "");
 
-  if (SuggestedCompletions.empty()) {
+  // When flag ends with '=' and there was no value completion, return empty
+  // string and fall back to the file autocompletion.
+  if (SuggestedCompletions.empty() && !Cur.endswith("=")) {
     // If the flag is in the form of "--autocomplete=-foo",
     // we were requested to print out all option names that start with "-foo".
     // For example, "--autocomplete=-fsyn" is expanded to "-fsyntax-only".

clang/test/Driver/autocomplete.c

@@ -115,3 +115,9 @@
 // Check if they can autocomplete values with coron
 // RUN: %clang --autocomplete=foo,bar,,,-fno-sanitize-coverage=,f | FileCheck %s -check-prefix=FNOSANICOVER-CORON
 // FNOSANICOVER-CORON: func
+
+// Clang should return empty string when no value completion was found, which will fall back to file autocompletion
+// RUN: %clang --autocomplete=-fmodule-file= | FileCheck %s -check-prefix=MODULE_FILE_EQUAL
+// MODULE_FILE_EQUAL-NOT: -fmodule-file=
+// RUN: %clang --autocomplete=-fmodule-file | FileCheck %s -check-prefix=MODULE_FILE
+// MODULE_FILE: -fmodule-file=