[analyzer] Add StdLibraryFunctions checker.

This checker does not emit reports, however it influences the analysis
by providing complete summaries for, or otherwise improving modeling of,
various standard library functions.

This should reduce the number of infeasible paths explored during analysis.
The custom function summary format used in this checker is superior to
body farms by causing less unnecessary state splits,
which would result in better analysis performance.

Differential Revision: https://reviews.llvm.org/D20811

llvm-svn: 284960