LIT MODE: execution flags and run configuration

[orchestr7]

LIT contains two functionalities that we need to support:

1. Support inline fixers for fix and warn plugin #221
2. RUN configuration:
   // RUN: %check_clang_tidy %s huawei-unspecified-array-size %t

In this task we need to support execution flags and run configuration on plugin level (but functionality should be common for plugins...)

RUN configuration should be ignored during the calculation of line numbers.

[orchestr7]

This issue should also cover cases when we would like to add something in the end of our execution command AFTER file names.

[petertrr]

Discussed and decided to do the following:
execFlags in save.toml can be extended with values from the file (comment format should be configurable via config file):

// RUN: extraFlagsBefore=stuff, extraFlagsAfter=stuff2

and then these variables can be used as placeholders:

execFlags = "-F $extraFlagsBefore $files $extraFlagsAfter"

or

[warn]
execFlags = "--fix $FLAGS $FILENAME $EXTRA_FLAGS"

[orchestr7]

LIT can contain SEVERAL RUN commands on the same file... Probably we should create an issue for it