[FPEnv][X86] Platform builtins edition: clang should get from the AST the...

[FPEnv][X86] Platform builtins edition: clang should get from the AST the metadata for constrained FP builtins

Currently clang is not correctly retrieving from the AST the metadata for
constrained FP builtins. This patch fixes that for the X86 specific builtins.

Differential Revision: https://reviews.llvm.org/D94614