rustc_lexer: Optimize shebang detection slightly

src/test/ui/parser/shebang/shebang-doc-comment.rs

@@ -0,0 +1,6 @@
+#!///bin/bash
+[allow(unused_variables)]
+//~^^ ERROR expected `[`, found doc comment
+
+// Doc comment is misinterpreted as a whitespace (regular comment) during shebang detection.
+// Even if it wasn't, it would still result in an error, just a different one.

src/test/ui/parser/shebang/shebang-doc-comment.stderr

@@ -0,0 +1,8 @@
+error: expected `[`, found doc comment `///bin/bash`
+  --> $DIR/shebang-doc-comment.rs:1:3
+   |
+LL | #!///bin/bash
+   |   ^^^^^^^^^^^ expected `[`
+
+error: aborting due to previous error
+