Make this test C instead of C++; making it C++ causes a failure on Linux

because clang can't parse stdio.h in C++ mode yet.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@52168 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/Lexer/digraph.cpp b/test/Lexer/digraph.c
similarity index 100%
rename from test/Lexer/digraph.cpp
rename to test/Lexer/digraph.c