Add MetaProcessor::awaitingMoreInput()

`MetaProcessor::awaitingMoreInput()` returns whether the collected input is
incomplete, either because it contains unbalanced braces or we found a
backslash-newline (the last seen token is a `\`).

Part of a patch series to fix ROOT-5219.

Co-authored-by: Axel Naumann <Axel.Naumann@cern.ch>

include/cling/MetaProcessor/MetaProcessor.h

@@ -123,6 +123,11 @@ namespace cling {
     /// input, resetting the continuation to a new line.
     void cancelContinuation() const;
 
+    ///\brief Returns whether we are waiting for more input, either because the
+    /// input contains imbalanced braces or a backslash-newline was seen (i.e.,
+    /// the last token was a `\`).
+    bool awaitingMoreInput() const;
+
     ///\brief Returns the number of imbalanced tokens seen in the current input.
     ///
     int getExpectedIndent() const;

lib/MetaProcessor/MetaProcessor.cpp

@@ -348,6 +348,11 @@ namespace cling {
     m_InputValidator->reset();
   }
 
+  bool MetaProcessor::awaitingMoreInput() const {
+    return m_InputValidator->getLastResult() ==
+           InputValidator::ValidationResult::kIncomplete;
+  }
+
   int MetaProcessor::getExpectedIndent() const {
     return m_InputValidator->getExpectedIndent();
   }