ClangPchManager: Allow errors in pch-s

There are cases when we can't avoid errors when we put all
system includes together. This might lead to not having any
pch-s which makes the indexing useless (too slow).

Change-Id: Icdbfdf916bbd0ed9b6cd18f9c976d3d4efb0e18f
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/tools/clangrefactoringbackend/source/symbolscollector.cpp

@@ -27,6 +27,7 @@
 
 #include <clang/Frontend/CompilerInstance.h>
 #include <clang/Frontend/FrontendActions.h>
+#include <clang/Lex/PreprocessorOptions.h>
 
 namespace ClangBackEnd {
 
@@ -100,6 +101,7 @@ newFrontendActionFactory(Factory *consumerFactory,
             bool BeginInvocation(clang::CompilerInstance &compilerInstance) override
             {
                 compilerInstance.getLangOpts().DelayedTemplateParsing = false;
+                compilerInstance.getPreprocessorOpts().AllowPCHWithCompilerErrors = true;
 
                 return clang::ASTFrontendAction::BeginInvocation(compilerInstance);
             }