Improve performance TokenizeWindowsCommandLine

Patcy by Takuto Ikuta.

This patch reduces lld link time of chromium's blink_core.dll in
component build.

Total size of input argument in .directives become nearly 300MB in the
build and calling many strchr and assert becomes bottleneck.

On my desktop machine, 4 times stats of the link time are like below.
Improved around 10%.

This patch
TotalSeconds : 13.4918885
TotalSeconds : 13.9474257
TotalSeconds : 13.4941082
TotalSeconds : 13.60779621
Avg : 13.63530465

master
TotalSeconds : 15.6938531
TotalSeconds : 15.7022508
TotalSeconds : 15.9567202
TotalSeconds : 14.5851505
Avg : 15.48449365

Differential Revision: https://reviews.llvm.org/D41590

llvm-svn: 321479