Add #pragma clang fp

Add #pragma clang fp

This adds the new pragma and the first variant, contract(on/off/fast).

The pragma has the same block scope rules as STDC FP_CONTRACT, i.e. it can be
placed at the beginning of a compound statement or at file scope.

Similarly to STDC FP_CONTRACT there is no need to use attributes.  First an
annotate token is inserted with the parsed details of the pragma.  Then the
annotate token is parsed in the proper contexts and the Sema is updated with
the corresponding FPOptions using the shared ActOn function with STDC
FP_CONTRACT.

After this the FPOptions from the Sema is propagated into the AST expression
nodes.  There is no change here.

I was going to add a 'default' option besides 'on/off/fast' similar to STDC
FP_CONTRACT but then decided against it. I think that we'd have to make option
uppercase then to avoid using 'default' the keyword.  Also because of the
scoped activation of pragma I am not sure there is really a need a for this.

Differential Revision: https://reviews.llvm.org/D31276

llvm-svn: 299470