CppTools: Allow interpreting ambiguous headers as C headers

...instead of C++ headers.

For the Clang Code Model this results in using "-x c-header" instead of
"-x c++-header".

This introduces a new option in Options > C++ > "Code Model" to
configure this.

Change-Id: I8a0ce8fa6155f5ef58743ebc7f1d0b500fbf6599
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/cpptools/cppcodemodelsettings.h

@@ -63,6 +63,9 @@ public:
     PCHUsage pchUsage() const;
     void setPCHUsage(PCHUsage pchUsage);
 
+    bool interpretAmbigiousHeadersAsCHeaders() const;
+    void setInterpretAmbigiousHeadersAsCHeaders(bool yesno);
+
     bool skipIndexingBigFiles() const;
     void setSkipIndexingBigFiles(bool yesno);
 
@@ -78,6 +81,7 @@ signals:
 
 private:
     PCHUsage m_pchUsage = PchUse_None;
+    bool m_interpretAmbigiousHeadersAsCHeaders = false;
     bool m_skipIndexingBigFiles = true;
     int m_indexerFileSizeLimitInMB = 5;
     ClangDiagnosticConfigs m_clangCustomDiagnosticConfigs;