Fix hash_template_argument_from_pretty_func for GCC 9

[jamiesnape]

Relates #13102.

__PRETTY_FUNCTION__ changed between GCC 7 and 9 as follows:

• GCC 7:

  drake::test::{anonymous}::NiceAnonEnumTemplate<(drake::test::<unnamed>::AnonEnum)0>

• GCC 9:

  drake::test::{anonymous}::NiceAnonEnumTemplate<drake::test::<unnamed>::AnonEnum::kFoo>
  

So it similar to Clang modulo {anonymous} and <unnamed> both mapping to (anonymous namespace):

drake::test::(anonymous namespace)::NiceAnonEnumTemplate<drake::test::(anonymous namespace)::AnonEnum::kFoo>

This therefore means that the drake::test::<unnamed>::AnonEnum is no longer discarded since it is not parenthesized and the difference between <unnamed> and {anonymous} matters, not least because the (unexpected) <> causes hash_template_argument_from_pretty_func to fail.

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This change is 

[jamiesnape]

@drake-jenkins-bot linux-focal-unprovisioned-gcc-bazel-experimental-release please

[jamiesnape]

+@jwnimmer-tri for feature review.

[jwnimmer-tri]

feature.

+@sherm1 for platform review per schedule, please.

Reviewed 2 of 2 files at r1.
Reviewable status: LGTM missing from assignee sherm1(platform) (waiting on @sherm1)

[sherm1]

Platform with one suggestion.

Reviewed 2 of 2 files at r1.
Reviewable status: 1 unresolved discussion (waiting on @jamiesnape)

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common/value.h, line 344 at r1 (raw file):

      }
    }
    if (*unnamed_iter == 0) {

minor: does this comparison have to be written bytewise? Consider instead:

if (strcmp(p, unnamed_spelling) == 0) {
  // ... output {anonymous} ...
  p += 9;  // Length of "<unnamed>".
}

That would be easier to read (assuming it works).

[jwnimmer-tri]

Reviewable status: complete! all discussions resolved, LGTM from assignees jwnimmer-tri(platform),sherm1(platform)

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common/value.h, line 344 at r1 (raw file):

Previously, sherm1 (Michael Sherman) wrote…

minor: does this comparison have to be written bytewise? Consider instead:

if (strcmp(p, unnamed_spelling) == 0) {
  // ... output {anonymous} ...
  p += 9;  // Length of "<unnamed>".
}

That would be easier to read (assuming it works).

strcmp is not constexpr
https://en.cppreference.com/w/c/string/byte/strcmp