When cross-compiling for Windows on Linux, fix mismatched types in nsMsgImapSearch.cpp. nsString::get() returns a char16ptr_t (with MOZ_USE_CHAR16_WRAPPER) and that causes a mismatch so we cast it to a const char16_t*. usAsciiCharSet is declared as a nsAutoString so we instead declare it as a char16_t* to avoid subsequent type mismatches.
Tag #1273
* Part 1: make conversion CTORs explicit.
Minor tweaks to conform with new(ish) static analysis rule which flags up implicit single-argument conversion constructors.
* Part 2: fix a couple of double-mRefCnt declaration errors.
* Part 3: Fix unsafe use of NS_ConvertASCIItoUTF16().
Tag #1273
