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

[dang]

Summary

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.

[zixuw]

Ha! Nice catch!

clang/lib/ExtractAPI/ExtractAPIConsumer.cpp

void MacroDefined(const Token &MacroNameToken,
     PendingMacros.emplace_back(MacroNameToken, MD);
   }
 
   // If a macro gets undefined at some point during preprocessing of the inputs
   // it means that it isn't an exposed API and we should therefore not add a
   // 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();
     });
   }
 
   void EndOfMainFile() override {
     for (auto &PM : PendingMacros) {

clang/test/ExtractAPI/macro_undefined.c

 // CHECK-NOT: warning:
 
 //--- input.h
 #define HELLO 1
 #define FUNC_GEN(NAME, ...) void NAME(__VA_ARGS__);
 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
 {
   "metadata": {
     "formatVersion": {
       "major": 0,
       "minor": 5,
       "patch": 3