[DRAFT] [backend] analyze procedures defined in changed headers

[VladimirMakaev]

With --changed-files-index, a changed header only selects the source file with the same name, so
procedures defined in headers (inline functions, in-class methods, templates) are not analyzed when
only the header changes.

// include/widget.h; changed files index: include/widget.h
inline int widget_get() {
  int* p = nullptr;
  return *p; // Infer missed: null dereference
}

// src/user.cpp, unchanged
#include "widget.h"
int use_widget() { return widget_get(); }

Now, for each procedure defined in a changed header, a captured source file that contains it is
analyzed too, and --incremental-analysis invalidates these procedures. With
--compilation-database, Infer warns about changed headers whose includers were not captured.

Limitations: such files are analyzed in full, so their unrelated issues are reported too, and the
file chosen for a header can change between versions, which reportdiff shows as fixed or
introduced issues.

Test plan

New build system tests incremental_analysis_change_header and differential_changed_header;
clang_compilation_db_headers also covers a partial index and a template used by two files. These,
the other clang_compilation_db* and incremental_analysis_* tests and the codetoanalyze tests
pass.