[ASan] Move flag validation from ParseFlagsFromString() to InitializeFlags().
[ASan] Restrict the set of flags overriden at activation.

Summary:
Change the way ASan flag parsing is performed at activation: instead
of overwriting all ASan and common flags from the activation string,
ASan will now only override a handful of whitelisted flags.
This is a first step towards making runtime flags immutable after
initialization. I plan to re-factor the activation stragegy to
the following one:

  - Parse commandline flags. Override the defaults from compile
    definition, env var, etc. Flags are immutable at this point.
  - Initiailize the runtime from commandline flags.
  - If ASan needs to be started deactivated, stash the runtime
    state in "asan_deactivated_flags" and deactivate the runtime.
  - When ASan is activated, override "asan_deactivated_flags" with
    activation flags, and use it to re-activate the runtime.

Test Plan: regression test suite

Reviewers: eugenis, kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6709