Some rustdoc diagnostic colors are missing

[ehuss]

As of #5995, some error messages from rustdoc are no longer getting colored.

Example code:

/// [BadLink]
/// ```
/// \
/// ```
pub fn f() {}

The result I get on nightly:

The result of cargo just before #5995:

Notice that the warning emitted by rustdoc is still colored, but the error: unknown start of token line caused by the backslash is not! So strange!

Tested on Windows and Mac.

cc @kennytm

[alexcrichton]

I believe the bug is here where the CLI --color argument isn't passed to all locations it's needed

[ehuss]

I think for the particular example above is specifically related to tokenization for syntax highlighting which goes through this line: https://github.com/rust-lang/rust/blob/60c846046ec26c9e5c1216a6e118cc2e69a2956c/src/libsyntax/parse/mod.rs#L67

The test one probably seems more useful to fix, though, since tokenization errors should be rare.

[ehuss]

This is now mostly fixed by rust-lang/rust#56884