[lldb/Expression] Make Language() const, NFC

Allow Language() to be called from const methods within UserExpression.

(cherry picked from commit 9e35498)

Author: vedantk

lldb/include/lldb/Expression/Expression.h

@@ -51,7 +51,9 @@ class Expression {
 
   /// Return the language that should be used when parsing.  To use the
   /// default, return eLanguageTypeUnknown.
-  virtual lldb::LanguageType Language() { return lldb::eLanguageTypeUnknown; }
+  virtual lldb::LanguageType Language() const {
+    return lldb::eLanguageTypeUnknown;
+  }
 
   /// Return the Materializer that the parser should use when registering
   /// external values.

lldb/include/lldb/Expression/UserExpression.h

@@ -194,7 +194,7 @@ class UserExpression : public Expression {
 
   /// Return the language that should be used when parsing.  To use the
   /// default, return eLanguageTypeUnknown.
-  lldb::LanguageType Language() override { return m_language; }
+  lldb::LanguageType Language() const override { return m_language; }
 
   /// Return the desired result type of the function, or eResultTypeAny if
   /// indifferent.