[Mips] Remove "single" from the list of valid MIPS float ABI names. Add
two new options –msingle-float and –mdouble-float. These options can be
used simultaneously with float ABI selection options (-mfloat-abi,
-mhard-float, -msoft-float). They mark whether a floating-point
coprocessor supports double-precision operations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179481 91177308-0d34-0410-b5e6-96231b3b80d8
Handle incompatible redeclarations of library builtins better.
Invalid redeclarations of valid explicit declarations shouldn't
take the same path as redeclarations of implicit declarations,
and invalid local extern declarations shouldn't foul things up
for everybody else.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179482 91177308-0d34-0410-b5e6-96231b3b80d8