Specify inline for isWhitespace in CommandLine.cpp

Patch by Takuto Ikuta.

In chromium's component build, there are many directive sections and
commandline parsing takes much time.
This patch is for speed up of lld in RelWithDebInfo build by forcing
inline heavily called isWhitespace function.

10 times link perf stats of blink_core.dll changed like below.

master:
TotalSeconds: 9.8764878
TotalSeconds: 10.1455242
TotalSeconds: 10.075279
TotalSeconds: 10.3397347
TotalSeconds: 9.8361665
TotalSeconds: 9.9544441
TotalSeconds: 9.89606869
TotalSeconds: 9.8877865
TotalSeconds: 10.0551879
TotalSeconds: 10.0492254
Avg: 10.01159047

with this patch:
TotalSeconds: 8.8696762
TotalSeconds: 9.1021585
TotalSeconds: 9.0233893
TotalSeconds: 9.1886175
TotalSeconds: 9.156954
TotalSeconds: 9.0978564
TotalSeconds: 9.1316824
TotalSeconds: 8.8354606
TotalSeconds: 9.2549431
TotalSeconds: 9.4473085
Avg: 9.11080465

llvm-svn: 322595