[SanitizerCoverage] Implement user-friendly -fsanitize-coverage= flags.

Summary:
Possible coverage levels are:
  * -fsanitize-coverage=func - function-level coverage
  * -fsanitize-coverage=bb - basic-block-level coverage
  * -fsanitize-coverage=edge - edge-level coverage

Extra features are:
  * -fsanitize-coverage=indirect-calls - coverage for indirect calls
  * -fsanitize-coverage=trace-bb - tracing for basic blocks
  * -fsanitize-coverage=trace-cmp - tracing for cmp instructions
  * -fsanitize-coverage=8bit-counters - frequency counters

Levels and features can be combined in comma-separated list, and
can be disabled by subsequent -fno-sanitize-coverage= flags, e.g.:
  -fsanitize-coverage=bb,trace-bb,8bit-counters -fno-sanitize-coverage=trace-bb
is equivalient to:
  -fsanitize-coverage=bb,8bit-counters

Original semantics of -fsanitize-coverage flag is preserved:
  * -fsanitize-coverage=0 disables the coverage
  * -fsanitize-coverage=1 is a synonym for -fsanitize-coverage=func
  * -fsanitize-coverage=2 is a synonym for -fsanitize-coverage=bb
  * -fsanitize-coverage=3 is a synonym for -fsanitize-coverage=edge
  * -fsanitize-coverage=4 is a synonym for -fsanitize-coverage=edge,indirect-calls

Driver tries to diagnose invalid flag usage, in particular:
  * At most one level (func,bb,edge) must be specified.
  * "trace-bb" and "8bit-counters" features require some level to be specified.

See test case for more examples.

Test Plan: regression test suite

Reviewers: kcc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D9577

llvm-svn: 236790