[Parser] Suppress -Wempty-translation-unit if this is a header file

[sammccall]

Summary

This is motivated by tooling (clangd, libclang etc) - headers without
declarations are legitimate even if they're not valid TUs.

The other use -x c-header cases (PCH/modules) are nonstandard anyway and this
warning doesn't seem necessary there either.

clang/lib/Parse/Parser.cpp

 /// Note that in C, it is an error if there is no first declaration.
 bool Parser::ParseFirstTopLevelDecl(DeclGroupPtrTy &Result) {
   Actions.ActOnStartOfTranslationUnit();
 
   // C11 6.9p1 says translation units must have at least one top-level
   // declaration. C++ doesn't have this restriction. We also don't want to
   // complain if we have a precompiled header, although technically if the PCH
   // is empty we should still emit the (pedantic) diagnostic.
+  // If the main file is a header, we're only pretending it's a TU; don't warn.
   bool NoTopLevelDecls = ParseTopLevelDecl(Result, true);
   if (NoTopLevelDecls && !Actions.getASTContext().getExternalSource() &&
-      !getLangOpts().CPlusPlus)
+      !getLangOpts().CPlusPlus && !getLangOpts().IsHeaderFile)
     Diag(diag::ext_empty_translation_unit);
 
   return NoTopLevelDecls;
 }
 
 /// ParseTopLevelDecl - Parse one top-level declaration, return whatever the
 /// action tells us to.  This returns true if the EOF was encountered.
 ///

clang/test/Parser/empty-translation-unit.c

 // RUN: %clang_cc1 -fsyntax-only -std=c99 -pedantic -W -verify %s
+// RUN: %clang_cc1 -fsyntax-only -x c-header -std=c99 -pedantic-errors -W %s
 // RUN: %clang_cc1 -fsyntax-only -x c++ -std=c++03 -pedantic-errors -W %s
 
 #include "completely-empty-header-file.h"
 // no-warning -- an empty file is OK
 
 #define A_MACRO_IS_NOT_GOOD_ENOUGH 1
 
-// In C we should get this warning, but in C++ we shouldn't.
+// In C we should get this warning, but in C++ (or a header) we shouldn't.
 // expected-warning{{ISO C requires a translation unit to contain at least one declaration}}