[clang][lex][minimizer] Ensure whitespace between squashed lines

The minimizer tries to squash multi-line macro definitions into single line. For that to work, contents of each line need to be separated by a space. Since we always strip leading whitespace on lines of a macro definition, the code currently tries to preserve exactly one space that appeared before the backslash.

This means the following code:

```
#define FOO(BAR) \
  #BAR           \
  baz
```

gets minimized into:

```
#define FOO(BAR) #BAR baz
```

However, if there are no spaces before the backslash on line 2:

```
#define FOO(BAR) \
  #BAR\
  baz
```

no space can be preserved, leading to (most likely) malformed macro definition:

```
#define FOO(BAR) #BARbaz
```

This patch makes sure we always put exactly one space at the end of line ending with a backslash.

Reviewed By: arphaman

Differential Revision: https://reviews.llvm.org/D119231