Keep track of whether declararions were loaded from a precompiled

Keep track of whether declararions were loaded from a precompiled
header or not via a new "PCHLevel" field in Decl. We currently use
this information to help CIndex filter out declarations that came from
a precompiled header (rather than from an AST file). Further down the
road, it can be used to help implement multi-level precompiled
headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@84267 91177308-0d34-0410-b5e6-96231b3b80d8