Add support for go:build style constraints

This doesn't account for the new syntax, but there was no syntax
parsing in the old version anyway, and the only user doesn't currently
care about semantics

ql/src/semmle/go/Comments.qll

@@ -189,7 +189,7 @@ private Comment getInitialComment(File f, int i) {
 }
 
 /**
- * A build constraint comment of the form `// +build ...`.
+ * A build constraint comment of the form `// +build ...` or `//go:build ...`.
  *
  * Examples:
  *
@@ -203,17 +203,23 @@ class BuildConstraintComment extends LineComment {
     // a line comment preceding the package declaration, itself only preceded by
     // line comments
     exists(File f, int i |
+      // correctness of the placement of the build constraint is not checked here;
+      // this is more lax than the actual rules for build constraints
       this = getInitialComment(f, i) and
       not getInitialComment(f, [0 .. i - 1]) instanceof BlockComment
     ) and
-    // comment text starts with `+build`
-    getText().regexpMatch("\\s*\\+build.*")
+    (
+      // comment text starts with `+build` or `go:build`
+      this.getText().regexpMatch("\\s*\\+build.*")
+      or
+      this.getText().regexpMatch("\\s*go:build.*")
+    )
   }
 
   override string getAPrimaryQlClass() { result = "BuildConstraintComment" }
 
   /** Gets the body of this build constraint. */
-  string getConstraintBody() { result = getText().splitAt("+build ", 1) }
+  string getConstraintBody() { result = getText().splitAt("build ", 1) }
 
   /** Gets a disjunct of this build constraint. */
   string getADisjunct() { result = getConstraintBody().splitAt(" ") }

ql/src/semmle/go/Files.qll

@@ -222,6 +222,8 @@ class ExtractedOrExternalFile extends Container, @file, Documentable, ExprParent
     exists(BuildConstraintComment bcc | this = bcc.getFile() |
       forex(string disjunct | disjunct = bcc.getADisjunct() |
         disjunct.splitAt(",").(Architecture).getBitSize() = bitSize
+        or
+        disjunct.splitAt("/").(Architecture).getBitSize() = bitSize
       )
     )
   }