Smarter HsType pretty-print for promoted datacons

Fix Trac #15898, by being smarter about when to print
a space before a promoted data constructor, in a HsType.
I had to implement a mildly tiresome function
    HsType.lhsTypeHasLeadingPromotionQuote
It has multiple cases, of course, but it's very simple.

The patch improves the error-message output in a bunch of
cases, and (to my surprise) actually fixes a bug in the
output of T14343 (Trac #14343), thus

  -  In the expression: _ :: Proxy '('( 'True,  'False),  'False)
  +  In the expression: _ :: Proxy '( '( 'True, 'False), 'False)

I discovered that there were two copies of the PromotionFlag
type (a boolean, with helpfully named data cons), one in
IfaceType and one in HsType.  So I combined into one,
PromotionFlag, and moved it to BasicTypes.  That's why
quite a few files are touched, but it's all routine.

0 files changed, 0 insertions, 0 deletions