[libtooling][clang-tidy] Fix crashing on rendering invalid SourceRanges

Invalid SourceRanges can occur generally if the code does not compile,
thus we expect clang error diagnostics.
Unlike `clang`, `clang-tidy` did not swallow invalid source ranges, but
tried to highlight them, and blow various assertions.

The following two examples produce invalid source ranges, but this is
not a complete list:

  void test(x); // error: unknown type name 'x'
  struct Foo {
    member; // error: C++ requires a type specifier for all declarations
  };

Thanks @whisperity helping me fix this.

Reviewed-By: xazax.hun

Differential Revision: https://reviews.llvm.org/D114254