[clang][extract-api] Undefining macros should not result in a crash

This fixes the situation where a undefining a not previously defined
macro resulted in a crash. Before trying to remove a definition from
PendingMacros we first check to see if the macro did indeed have a
previous definition.

Differential Revision: https://reviews.llvm.org/D123056

clang/lib/ExtractAPI/ExtractAPIConsumer.cpp

@@ -534,6 +534,11 @@ public:
   // macro definition for it.
   void MacroUndefined(const Token &MacroNameToken, const MacroDefinition &MD,
                       const MacroDirective *Undef) override {
+    // If this macro wasn't previously defined we don't need to do anything
+    // here.
+    if (!Undef)
+      return;
+
     llvm::erase_if(PendingMacros, [&MD](const PendingMacro &PM) {
       return MD.getMacroInfo()->getDefinitionLoc() ==
              PM.MD->getMacroInfo()->getDefinitionLoc();

clang/test/ExtractAPI/macro_undefined.c

@@ -19,6 +19,8 @@
 FUNC_GEN(foo)
 FUNC_GEN(bar, const int *, unsigned);
 #undef FUNC_GEN
+// Undefining a not previously defined macro should not result in a crash.
+#undef FOO
 
 //--- reference.output.json.in
 {