[analyzer] fix an issue on #include<..> when using scan-build with -isystem

[takeshi-yoshimura]

Summary

scan-build fails to perform static analysis if a hooked compilation uses
-isystem options in particular forms. It results in odd build errors,
which include<..>'ed files are not found even though the build before the
static analysis succeeds. This patch addresses a bug reported in PR13237:
https://llvm.org/bugs/show_bug.cgi?id=13237

The fault is a broken regular expression to extract paths after include
options string in ccc-analyzer. Specifically, the analyzer looks over the
path string after -isystem if we put a space between them. Note that scan-
build already can correctly parse the option when there are no spaces
between -isystem and the path (e.g., -isystem/path/to/dir/).

I also removed an FIXME near there since I do not think $ARGV[$i] can
overflow here. $ARGV overflows mean there are no strings after include
options. It prevents the compilation before static analysis from
succeeding, i.e., we cannot reach here.

test/Analysis/scan-build-isystem.h

+// dummy file
 No newline at end of file

test/Analysis/scan-build-isystem.c

+// RUN: perl %S/../../tools/scan-build/scan-build --use-analyzer %clang -o %t %clang_cc1 -isystem%S -fsyntax-only %s 2>&1
+// RUN: perl %S/../../tools/scan-build/scan-build --use-analyzer %clang -o %t %clang_cc1 -isystem %S -fsyntax-only %s 2>&1
+// ccc-analyzer has to correctly look up and include -isystem directory to avoid static analysis failures in scan-build.
+
+#include <scan-build-isystem.h>
+
+void f() {
+  int * i = 0;
+  *i = 1; // CHECK: Dereference of null pointer
+}

tools/scan-build/ccc-analyzer

if (defined $IgnoredOptionMap{$ArgKey}) {
     my $Cnt = $IgnoredOptionMap{$ArgKey};
     while ($Cnt > 0) {
       ++$i; --$Cnt;
     }
     next;
   }
 
   # Compile mode flags.
-  if ($Arg =~ /^-[D,I,U,isystem](.*)$/) {
+  if ($Arg =~ /^-[D,I,U](.*)$/ || $Arg =~ /^-isystem(.*)$/) {
     my $Tmp = $Arg;
     if ($1 eq '') {
-      # FIXME: Check if we are going off the end.
       ++$i;
       $Tmp = $Arg . $ARGV[$i];
     }
     push @CompileOpts,$Tmp;
     next;
   }
 
   if ($Arg =~ /^-m.*/) {