auto merge of #15160 : alexcrichton/rust/remove-f128, r=brson

The f128 type has very little support in the compiler and the feature is
basically unusable today. Supporting half-baked features in the compiler can be
detrimental to the long-term development of the compiler, and hence this feature
is being removed.

diff --cc src/librustc/front/feature_gate.rs
Simple merge

diff --cc src/librustc/middle/resolve.rs
Simple merge

diff --cc src/librustc/middle/trans/debuginfo.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/ast_util.rs
Simple merge

diff --cc src/libsyntax/ext/quote.rs
Simple merge