Allow the preprocessor to be constructed without performing target-

Allow the preprocessor to be constructed without performing target-
and language-specific initialization. Use this to allow ASTUnit to
create a preprocessor object *before* loading the AST file. No actual
functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138983 91177308-0d34-0410-b5e6-96231b3b80d8