[Doc] Fix errors in SIL.rst.

[dan-zheng]

Fix "Title underline too short" errors:

Warning, treated as error:
/Users/danielzheng/swift-dev/swift/docs/SIL.rst:5604:Title underline too short.

differentiable_function
`````````````````
make: *** [/Users/danielzheng/swift-dev/build/Xcode-ReleaseAssert+swift-DebugAssert/swift-macosx-x86_64/docs/CMakeFiles/docs_html] Error 2
Command /bin/sh failed with exit code 2

Todo: install Sphinx on TensorFlow CI machines to catch these errors.

[rxwei]

Feel free to merge without running CI.

[dan-zheng]

Installed Sphinx on all three CI machines to catch .rst errors in the future.

$ sudo apt-get install python3-sphinx
$ brew install sphinx-doc

[dan-zheng]

Feel free to merge without running CI.

I don't plan to land other patches soon so I'll start CI.

[dan-zheng]

@swift-ci Please test tensorflow

[burmako]

Looks like another legitimate error, also in an .rst file:

Warning, treated as error:
/Users/swiftci/jenkins/workspace/swift-PR-TensorFlow-macOS/swift/docs/ARCOptimization.rst:100:Pygments lexer name 'sil' is not known

[dan-zheng]

macOS CI now fails with:

Warning, treated as error:
/Users/swiftci/jenkins/workspace/swift-PR-TensorFlow-macOS/swift/docs/ARCOptimization.rst:100:Pygments lexer name 'sil' is not known

Investigating briefly, it seems others have ran into it here. If it's not easily fixable, I'll uninstall sphinx from the macOS CI machine.

---

EDIT: I tried sphinx installed via pip3 install sphinx and brew install sphinx-doc on the macOS machine but both failed with the same error: Pygments lexer name 'sil' is not known. Uninstalling sphinx to unblock progress - it seems to run fine on Ubuntu CI at least.

[dan-zheng]

@swift-ci Please test tensorflow macOS

[dan-zheng]

macOS CI fails with a super weird error:

/Users/swiftci/jenkins/workspace/swift-PR-TensorFlow-macOS/swift/test/AutoDiff/differentiable_function_silgen.swift:35:15: error: CHECK-AST: expected string not found in input
// CHECK-AST: (function_conversion_expr implicit type='@differentiable (Float) -> (Float)'
              ^
<stdin>:69:110: note: scanning from here
 (func_decl range=[SOURCE_DIR/test/AutoDiff/differentiable_function_silgen.swift:23:1 - line:25:1] "apply()" interface type='() -> ()' access=internal
                                                                                                             ^
<stdin>:79:2: note: possible intended match here
 (differentiable_function implicit type='@differentiable (Float) -> Float' location=SOURCE_DIR/test/AutoDiff/differentiable_function_silgen.swift:24:18 range=[SOURCE_DIR/test/AutoDiff/differentiable_function_silgen.swift:24:18 - line:24:18]
 ^

I ssh'd into the macOS CI machine and found that the RUN line produces the following -dump-ast output:

(function_conversion_expr implicit type
='@differentiable (Float) -> (Float)'

Note the newline after (function_conversion_expr implicit type: it is what causes the FileCheck error. I believe it is fixed in 8397226, verified on the macOS CI machine. The error probably leaked through because macOS CI was down for #27471.

[dan-zheng]

@swift-ci Please test tensorflow