Eliminate Sema::KnownFunctionIDs, so that Sema doesn't end up pulling

in a bunch of declarations from the PCH file. We're down to loading
very few declarations in Carbon-prefixed "Hello, World!":

*** PCH Statistics:
  6/20693 types read (0.028995%)
  7/59230 declarations read (0.011818%)
  50/44914 identifiers read (0.111324%)
  0/32954 statements read (0.000000%)
  5/6187 macros read (0.080815%)

llvm-svn: 69825