[libFuzzer] Update documentation regarding MSan.

[morehouse]

Summary

-fsanitize=fuzzer,memory now works out-of-the-box.

[morehouse]

Ran fuzzer-test-suite's test-everything.sh with -fsanitize=memory,fuzzer and all MSan reports came from the fuzz target, not libFuzzer. I think it's time to document the new capability.

[kcc]

all MSan reports came from the fuzz target, not libFuzzer

It may be more subtle than this. Did you investigate the msan reports?

[morehouse]

Benchmarks easy to discern the crash cause:

• boringssl - Finds same use-after-free as ASan.
• c-ares - Finds same crash as vanilla libFuzzer.
• freetype - Finds the target line.
• guetzli - False positive (fuzz target uses std::map).
• harfbuzz - Hits time limit without crashing.
• lcms - No crashes after several days.
• libjpeg-turbo - Finds the target line.
• openssl-1.0.1f - Finds Heartbleed.
• pcre2 - Finds same buffer overflow as ASan.

Benchmarks with new crash stack traces under MSan. Would take more effort to determine cause:

• json
• libarchive
• libpng
• libssh
• libxml
• openssl-1.0.2d
• proj4
• re2
• woff2

I could investigate each of the new crashes, but that's more time than I'd like to spend on this. Maybe I will just change the documentation to say that MSan support is experimental.

[morehouse]

• Support MSan experimentally.

[kcc]

LGTM

llvm/docs/LibFuzzer.rst

 
 Fuzzer Usage
 ------------
 
 Recent versions of Clang (starting from 6.0) include libFuzzer, and no extra installation is necessary.
 
 In order to build your fuzzer binary, use the `-fsanitize=fuzzer` flag during the
 compilation and linking. In most cases you may want to combine libFuzzer with
-AddressSanitizer_ (ASAN), UndefinedBehaviorSanitizer_ (UBSAN), or both::
+AddressSanitizer_ (ASAN), UndefinedBehaviorSanitizer_ (UBSAN), or both.  You can
+also build with MemorySanitizer_ (MSAN), but support is experimental::
 
    clang -g -O1 -fsanitize=fuzzer                         mytarget.c # Builds the fuzz target w/o sanitizers
    clang -g -O1 -fsanitize=fuzzer,address                 mytarget.c # Builds the fuzz target with ASAN
    clang -g -O1 -fsanitize=fuzzer,signed-integer-overflow mytarget.c # Builds the fuzz target with a part of UBSAN
+   clang -g -O1 -fsanitize=fuzzer,memory                  mytarget.c # Builds the fuzz target with MSAN
 
 This will perform the necessary instrumentation, as well as linking with the libFuzzer library.
 Note that ``-fsanitize=fuzzer`` links in the libFuzzer's ``main()`` symbol.
 
 If modifying ``CFLAGS`` of a large project, which also compiles executables
 requiring their own ``main`` symbol, it may be desirable to request just the
 instrumentation without linking::
 
    clang -fsanitize=fuzzer-no-link mytarget.c
 
 Then libFuzzer can be linked to the desired driver by passing in
 ``-fsanitize=fuzzer`` during the linking stage.
 
-Using MemorySanitizer_ (MSAN) with libFuzzer is possible too, but tricky.
-The exact details are out of scope, we expect to simplify this in future
-versions.
-
 .. _libfuzzer-corpus:
 
 Corpus
 ------
 
 Coverage-guided fuzzers like libFuzzer rely on a corpus of sample inputs for the
 code under test.  This corpus should ideally be seeded with a varied collection
 of valid and invalid inputs for the code under test; for example, for a graphics