[Analyzer] Add support for displaying cross-file diagnostic paths in HTML output

This change adds support for cross-file diagnostic paths in html output. If the
diagnostic path is not cross-file, there is no change in the output.

Patch by Vlad Tsyrklevich!

Differential Revision: https://reviews.llvm.org/D30406

llvm-svn: 309968
diff --git a/clang/test/Coverage/html-multifile-diagnostics.h b/clang/test/Coverage/html-multifile-diagnostics.h
new file mode 100644
index 0000000..4a99ff0
--- /dev/null
+++ b/clang/test/Coverage/html-multifile-diagnostics.h
@@ -0,0 +1,3 @@
+void f1(int *ptr) {
+  *ptr = 0;
+}