Generalize -Wempty-body: warn when statement body is empty (closes: PR11329)

* if, switch, range-based for: warn if semicolon is on the same line.
* for, while: warn if semicolon is on the same line and either next
statement is compound statement or next statement has more
indentation.

Replacing the semicolon with {} or moving the semicolon to the next
line will always silence the warning.

Tests from SemaCXX/if-empty-body.cpp merged into SemaCXX/warn-empty-body.cpp.

llvm-svn: 150515