[llvm-cov][CoverageViewHTML] add style for selected line

Highlight the selected line. One major benefit is, if the page is
relatively short, the line pointed by #Ln in URL may not be on top, and
this highlight can help quickly locate the line.

Author: whentojump

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

@@ -246,6 +246,9 @@ tr:hover {
 tr:last-child {
   border-bottom: none;
 }
+tr:has(> td >a:target) > td.code > pre {
+  background-color: #ffa;
+}
 )";
 
 const char *EndHeader = "</head>";