C++: Expose PCH file creation as usage in QL

Author: jketema

cpp/ql/lib/cpp.qll

@@ -15,6 +15,7 @@
 
 import Customizations
 import semmle.code.cpp.File
+import semmle.code.cpp.PchFile
 import semmle.code.cpp.Linkage
 import semmle.code.cpp.Location
 import semmle.code.cpp.Compilation

cpp/ql/lib/semmle/code/cpp/PchFile.qll

@@ -0,0 +1,23 @@
+/**
+ * Provides the `PchFile` class representing precompiled header (PCH) files created and
+ * used during the build process.
+ */
+
+import semmle.code.cpp.File
+
+/**
+ * A precompiled header (PCH) file created during the build process.
+ */
+class PchFile extends @pch {
+  string toString() { result = "PCH for " + this.getHeaderFile() }
+
+  /**
+   * Gets the header file from with the PCH file was created.
+   */
+  File getHeaderFile() { pch_creations(this, _, result) }
+
+  /**
+   * Gets a source file that includes the PCH.
+   */
+  File getAUse() { pch_uses(this, _, result) }
+}