Port https://github.com/dotnet/corert/pull/629 to CoreCLR

[bbowyersmyth]

fixed (char* chPtr1 = strA)

compiles to

string str = strA;
char* chPtr1 = (char*) str;
if ((IntPtr) chPtr1 != IntPtr.Zero)
    chPtr1 += RuntimeHelpers.OffsetToStringData;

but string already has knowledge of where the offset is. This also brings these fixed statements in line with the rest of the file.

[stephentoub]

LGTM