[llvm] Remove out-of-date fixme from SmallVectorMemoryBuffer

[jansvoboda11]

Summary

This fixme first appeared in the codebase with the introduction of ObjectMemoryBuffer in rG93de2a12a36feaf1e6d8ff28c76db9b6cda4e844, but the constructor appears to never have been templated.

[dexonsmith]

LGTM.

llvm/include/llvm/Support/SmallVectorMemoryBuffer.h

 /// This class enables efficient construction of MemoryBuffers from SmallVector
 /// instances. This is useful for MCJIT and Orc, where object files are streamed
 /// into SmallVectors, then inspected using ObjectFile (which takes a
 /// MemoryBuffer).
 class SmallVectorMemoryBuffer : public MemoryBuffer {
 public:
   /// Construct an SmallVectorMemoryBuffer from the given SmallVector
   /// r-value.
-  ///
-  /// FIXME: It'd be nice for this to be a non-templated constructor taking a
-  /// SmallVectorImpl here instead of a templated one taking a SmallVector<N>,
-  /// but SmallVector's move-construction/assignment currently only take
-  /// SmallVectors. If/when that is fixed we can simplify this constructor and
-  /// the following one.
   SmallVectorMemoryBuffer(SmallVectorImpl<char> &&SV)
       : SV(std::move(SV)), BufferName("<in-memory object>") {
     init(this->SV.begin(), this->SV.end(), false);
   }
 
   /// Construct a named SmallVectorMemoryBuffer from the given
   /// SmallVector r-value and StringRef.
   SmallVectorMemoryBuffer(SmallVectorImpl<char> &&SV, StringRef Name)