Clang: Correct default C standards for ancient versions

C11 was made default in LLVM commit ab506adf7d3ced6abcaf42f92de3d6cd15fa19e8,
released in 3.5.2.
C99 was made default in LLVM commit 17f76e04d244c80e70f1c81c94d4524b53d9772d,
released in 2.1. It was flipped a few times between C89 and C99 during the 2.1
cycle, but the C89 default never made it into a release.