Formatter: Get bit tests in ifs right.

It's generally not possible to know if 'a' '*' 'b' is a multiplication
expression or a variable declaration with a purely lexer-based approach. The
formatter currently uses a heuristic that classifies this token sequence as a
multiplication in rhs contexts (after '=' or 'return') and as a declaration
else.

Because of this, it gets bit tests in ifs, such as "if (a & b)" wrong. However,
declarations in ifs always have to be followed by '=', so this patch changes
the formatter to classify '&' as an operator if it's at the start of an if
statement.

Before:
  if (a& b)
  if (int* b = f())

Now:
  if (a & b)
  if (int* b = f())



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172731 91177308-0d34-0410-b5e6-96231b3b80d8